A Grosvenor Gambit is an intentional misplay of a hand aimed to psychologically disturb one's opponents. It can be employed by declarer and defenders alike, but this description focuses on the defense.
This example comes from the original Bridge World article
describing fictional occurrences of the gambit.
South declares 6
and West leads his two top clubs. After ruffing the second, South cashes his two top spades and ruffs a third spade with dummy's J.
East could overruff with the Q
to immediately set the contract, but discards instead. Now South incorrectly assumes that West has the queen, and cashes the top trumps. East scores the Q
after all, and presumably upsets South with his antics.