 The word is being used facetiously based on 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.