They are sites who offer the possibility to show your cards. So...to show or to muck? I usually show my bluffs to give a false image. Sometimes i show very god hands (four of a kind, royal flush) but this times are rare:)) So...when u have a big pair or u make a big bet and the others fold but ask u to show what is the best move? pay and see or take a look what good cards i have?:)
