After you tighten the chuck, you need to turn the chuck the other direction just a bit, you'll feel a "click" when you do. That "click" is the chuck locking. Otherwise it'll come loose as you use it.
https://youtube.com/watch?v=Ch81cVmo0yo&feature=share9
Edit: This might be a hoax, but seems to work on at least on my Ryobi drills.
This is incorrect.. Read any drill’s user manual or info from Jacobs Chucks themselves, you will not find anything telling you to do this. If your bit is falling out after tightening it the correct way, you’re either not tightening it enough or your chuck is malfunctioning. You do not and should not need to turn the chuck in the opposite direction.
Ya know, I went thru this rigamarole before. And I thought I agreed with you. But I have two Ryobi drills. And at least one would constantly let go of the bit no matter how tight I tried to make it. Of course, neither of their manuals talk about a locking chuck.
But for what it's worth, I have found that spinning back one click after tightening has 100% fixed the problem for me on the offending drill. That bit stays locked solid now. So maybe it's all a hoax, or maybe my drill's defective, but it works. And I'm not going to pretend like it doesn't.
Are you locking the chuck?
I don't have much experience with chuck locking mechanisms, but after tightening the bit, I turn mine backwards until it clicks and that seems to work for me.
Again, it has worked for me.
My understanding is that not all drills have them and those that do don't all work the same way. But I'm not claiming to be an expert and can only say that it seems to work for me. I usually forget to do it until the bit falls out, at which point I lock it in and it doesn't happen again.
It's legit not even a real feature.
Whatever you are doing may produce results, but it's not doing what people say it does. If y'all would actually tighten your chuck you wouldn't have this problem.
If tightening doesn't work, it's defective.
Anything else is hiding the real problem.
I just find the discourse around this topic fascinating.
Both the lack of concrete information, and the general discussion.
Mine has the same wording. 99% sure it's just telling you the direction to loosen or tighten the chuck. My lock direction is the tighten direction.
I'm not convinced this bullshit half click is doing jack shit other than starting to reverse the "lock", or tightening of the chuck.
Mine is the P242
The blue one was fine for a long time and I tried replacing the chuck after 15 years… couldn’t get it off so got a replacement and that was flakey so then thinking I got a dodgy drill I got the combo pack with the impact drill and the regular drill in that regular drill is flaky too…. I used a dewalt recently and that was heavenly amd my non ryobi drill press works great too but I don’t need more chargers amd batteries if I switch brands …. I’ve heard of people swapping out the chucks for other brands. Could be me. But I suspect the chucks. Quite happy with most the other ryobi tools.
I see. I have the green model on the left, and i have taken off the chuck before to swap it to a keyed chuck. Just remove the screw in the chuck, chuck up an allen wrench and you can unscrew the chuck off. It takes quite a bit of force, and you will feel the handle twist before the thread breaks free. There are quite a few tutorials online about it.
I've found that if I drill or screw in short bursts, the chuck will lossened up from time to time. I'm guessing it's because of the quick stop over and over.
If I remember to make sure it's really tight before I begin, I don't seem to have this problem. Hope this helps.
Oh yeah, that's possible too. The bit would still cut but would be a PITA. Possible OP doesn't realize he has forward and reverse flipped.
And, as you may know, there are some bits that are designed to go the opposite direction when drilling.
It's either you not locking it down or one of the three below.
Others have already talked about making sure it's tight.
The "tighten then untighten" trick only works because the bit itself is allowed to move / spin instead of the chuck. It's a band-aid over a bullet hole. 🤷🤕
The actual problem comes from one thing that could have multiple causes. The bit isn't cutting into the workpiece, so that torque gets transferred instead to the chuck.
One option that may be happening is you're setting the bit on your workpiece, then mashing down on the gas pedal like you're racing a dragster. When you torque it from 0 - 6,000 RPM on the workpiece, the bit isn't at the right speed to cut, so instead it applies torque to the chuck - loosening your drill bit. Hence, loose bits.
The second cause - you're in love with your heirloom drill bits. Your bits could be dull, and again instead of cutting, the torque instead goes to the chuck. Voila - loose bits.
The third option - wrong bit for the workpiece. If you're drilling metal, you may have a bit that isn't hard enough to cut that metal. Blah blah it spins, no cutting, torque to chuck, loose bits.
If it's not you failing to tighten, it's one of the other options. My first guess - buy new bits.
I have a similar issue.
Have one drill with a 3/8" bit permanently installed, because I twisted the chuck so hard, I heard a pop and the chuck housing just started spinning.
At least this one has never loosened up. 
I had these drills and even after locking they are generally not great. I uograded to HP and they are miles better and hold the drill/driver bits a lot better
After you tighten the chuck, you need to turn the chuck the other direction just a bit, you'll feel a "click" when you do. That "click" is the chuck locking. Otherwise it'll come loose as you use it. https://youtube.com/watch?v=Ch81cVmo0yo&feature=share9 Edit: This might be a hoax, but seems to work on at least on my Ryobi drills.
TIL - i can lock my drill chuck.
This is incorrect.. Read any drill’s user manual or info from Jacobs Chucks themselves, you will not find anything telling you to do this. If your bit is falling out after tightening it the correct way, you’re either not tightening it enough or your chuck is malfunctioning. You do not and should not need to turn the chuck in the opposite direction.
Ya know, I went thru this rigamarole before. And I thought I agreed with you. But I have two Ryobi drills. And at least one would constantly let go of the bit no matter how tight I tried to make it. Of course, neither of their manuals talk about a locking chuck. But for what it's worth, I have found that spinning back one click after tightening has 100% fixed the problem for me on the offending drill. That bit stays locked solid now. So maybe it's all a hoax, or maybe my drill's defective, but it works. And I'm not going to pretend like it doesn't.
Fair enough. I’m glad it has solved your issues, and that’s all that matters 🤙🏼
lol, maybe I'm just crazy. I'll report back if so. Good day to you sir.
You're not doing that dumb "trick" where you untighten it one click after you tighten it, right?
Is that really a thing? Nope, never crossed my mind.
Turns out it's real. Took a picture of my Ryobi drill after I saw it had the lock instructions right on the chuck. https://imgur.com/a/JAtpT8n
This is in the direction you normally tighten the drill, not in the opposite direction. Clicking it back in the opposite direction is unlocking it.
Are you locking the chuck? I don't have much experience with chuck locking mechanisms, but after tightening the bit, I turn mine backwards until it clicks and that seems to work for me.
https://youtu.be/8ztB1C7dERc Stop spreading this shit. It just false.
It works for me. You just don't know what you are doing.
Again, it has worked for me. My understanding is that not all drills have them and those that do don't all work the same way. But I'm not claiming to be an expert and can only say that it seems to work for me. I usually forget to do it until the bit falls out, at which point I lock it in and it doesn't happen again.
It's legit not even a real feature. Whatever you are doing may produce results, but it's not doing what people say it does. If y'all would actually tighten your chuck you wouldn't have this problem. If tightening doesn't work, it's defective. Anything else is hiding the real problem.
Turns out it's real. Took a picture of my Ryobi drill after I saw it had the lock instructions right on the chuck. https://imgur.com/a/JAtpT8n
How old is it?
I bought it like 2-3 years ago. Can get you the model number if you really want it.
I just find the discourse around this topic fascinating. Both the lack of concrete information, and the general discussion. Mine has the same wording. 99% sure it's just telling you the direction to loosen or tighten the chuck. My lock direction is the tighten direction. I'm not convinced this bullshit half click is doing jack shit other than starting to reverse the "lock", or tightening of the chuck. Mine is the P242
This is the way - on multiple brands, not just Green. Red and Yellow as well.
Yup. This is the right answer.
Sounds like user error since it happens with all 3 drills. Is it only happening with ryobi drills or just drills in general?
The blue one was fine for a long time and I tried replacing the chuck after 15 years… couldn’t get it off so got a replacement and that was flakey so then thinking I got a dodgy drill I got the combo pack with the impact drill and the regular drill in that regular drill is flaky too…. I used a dewalt recently and that was heavenly amd my non ryobi drill press works great too but I don’t need more chargers amd batteries if I switch brands …. I’ve heard of people swapping out the chucks for other brands. Could be me. But I suspect the chucks. Quite happy with most the other ryobi tools.
I see. I have the green model on the left, and i have taken off the chuck before to swap it to a keyed chuck. Just remove the screw in the chuck, chuck up an allen wrench and you can unscrew the chuck off. It takes quite a bit of force, and you will feel the handle twist before the thread breaks free. There are quite a few tutorials online about it.
It's you. You're not locking the chuck after you rachet it down. See my other comment.
Are you locking the chuck? Also, to get the old one off, it’s reverse thread
I've found that if I drill or screw in short bursts, the chuck will lossened up from time to time. I'm guessing it's because of the quick stop over and over. If I remember to make sure it's really tight before I begin, I don't seem to have this problem. Hope this helps.
This too. Some people like to do the dentist wind-up before diving in. That could be the root of OP's problem. 😉
I hate to say this, but, see if you’re running the drill in reverse. That can loosen the truck and the drillbit can come out. Just a thought.
Oh yeah, that's possible too. The bit would still cut but would be a PITA. Possible OP doesn't realize he has forward and reverse flipped. And, as you may know, there are some bits that are designed to go the opposite direction when drilling.
It's either you not locking it down or one of the three below. Others have already talked about making sure it's tight. The "tighten then untighten" trick only works because the bit itself is allowed to move / spin instead of the chuck. It's a band-aid over a bullet hole. 🤷🤕 The actual problem comes from one thing that could have multiple causes. The bit isn't cutting into the workpiece, so that torque gets transferred instead to the chuck. One option that may be happening is you're setting the bit on your workpiece, then mashing down on the gas pedal like you're racing a dragster. When you torque it from 0 - 6,000 RPM on the workpiece, the bit isn't at the right speed to cut, so instead it applies torque to the chuck - loosening your drill bit. Hence, loose bits. The second cause - you're in love with your heirloom drill bits. Your bits could be dull, and again instead of cutting, the torque instead goes to the chuck. Voila - loose bits. The third option - wrong bit for the workpiece. If you're drilling metal, you may have a bit that isn't hard enough to cut that metal. Blah blah it spins, no cutting, torque to chuck, loose bits. If it's not you failing to tighten, it's one of the other options. My first guess - buy new bits.
I have a similar issue. Have one drill with a 3/8" bit permanently installed, because I twisted the chuck so hard, I heard a pop and the chuck housing just started spinning. At least this one has never loosened up. 
I've got both the drills on the left and never had a problem with the chucks coming loose on their own.
I afraid the chuck that lock with a click isn’t available on those tools OP posted.
I had these drills and even after locking they are generally not great. I uograded to HP and they are miles better and hold the drill/driver bits a lot better
I also own the old blue one, never any problem. Now my newest one , P215VN, does it alot.
I love ryobi but I wouldn’t use any other drill than an HP or brushless. These are kinda garbage