To underplay is to play a strong poker hand more weakly than it deserves, usually when you have a very good chance of winning. A player holding a favourable hand slows things down: betting small, or taking time before betting or calling, as if still deciding whether to continue.
The aim is a bigger pot. Push all-in straight away and most opponents without a monster of their own will fold, whether they read you as strong or simply won’t risk their chips on a guess, so the pot stays small. Even if you win, you win very little. By betting lightly and letting the hand develop, the underplaying player encourages others to believe they still have a chance, so they keep putting chips in.
Underplay in online poker
Without faces to read, timing and bet sizing are an online player’s disguise, and slow-playing is a normal part of strategy. Poker security teams still know the pattern well: persistently passive play between the same accounts, hand after hand, is one of the signs reviewed in collusion checks.