You can choose to check with either the mouse or the keyboard (K). After a person raises or if you are not in the blind, the mouse selection is removed but the keyboard selection of check still can be used. This allows you into the pot without having to put any of your own money. If you lose, the opponent wins their money back. If you win, you will get the money without risking yours. This doesn't work on all in bets; instead it acts as if the all in never happened.