Slow playing (also called sandbagging or trapping) is deceptive play in poker that is roughly the opposite of bluffing: betting weakly or passively with a strong holding rather than betting aggressively with a weak one. The flat call is one such play. The objective of the passive slow play is to lure opponents into a pot who might fold to a raise, or to cause them to bet more strongly than they would if the player had played aggressively (bet or raised). Slow playing sacrifices protection against hands that may improve and risks losing the pot-building value of a bet if the opponent also checks.
David Sklansky defines the following conditions for profitable slow plays:
A player must have a very strong hand.
The free card or cheap card the player is allowing to his opponents must have good possibilities of making them a second-best hand.
That same free card must have little chance of giving an opponent a better hand or even giving them a draw to a better hand on the next round with sufficient pot odds
to justify a call.
The player must believe that he will drive out opponents by showing aggression, but can win a big pot if the opponents stay in the pot.
The pot must not yet be very large