The Alcatraz coup is an illegal method of learning about the opponents' cards in contract bridge. It is not a true coup; the word is being used facetiously in conjunction with the name of the former Alcatraz penitentiary. The "coup" consists of a deliberate revoke by declarer, causing the next player to reveal whether he holds the key card whose location is sought. The declarer then corrects the revoke (which is allowed without penalty if done soon enough) and the defender may change his play, but declarer now knows about the key card and can choose how to finesse accordingly.
Consider this layout of the club suit:
South wants three tricks from clubs but the opponents have been successful in concealing the location of the Q. South calls for the J from dummy, and East follows suit with the 3. South now perpetrates the Alcatraz Coup by discarding from a different suit, thus committing a revoke. Then:
If West follows suit with a small card, South corrects his revoke by replacing his discard with the 8. Having scored the J, South now cashes the K and later the A.
If West plays the Q on dummy's J, South corrects his revoke by replacing his discard with the K. West can now take back his Q, of course, but with the position exposed South confidently finesses West for the Q.