You must play if you can
If this rule is in place, you must play some of your cards if you can. This rule is often used to prevent players from saving up all their cards, and then play them all in one go.
When the game determines if you can play or not, it only looks for very simple moves. You are never forced to find complicated combinations.
If a game has the rule if you don't play, draw until you can, then this rule is always set, too.
If this rule is in place, and you run out of time while you haven't played yet, then the computer will play something for you.
Games with this rule
The games Shanghai and Windmill use this rule; the other built-in games in RRRummy don't.
In some Rummy type games, like Shanghai, you have to play something if you can.