The Vienna coup is an unblocking technique in contract bridge made in preparation for a squeeze play. It is so named because it was originally published by James Clay (1804-1873) after observing it being executed in the days of whist by "the greatest player in Vienna" — identity unknown.
Examples
On the play of the ♣A by South, East is squeezed but can escape by throwing a small heart. Although the ♥Q is now set up, South must next play either the ♥Q, ♥2 or ♠4 to be won in dummy and has no entry back to cash it; he must now lose a spade to East.
However, if instead, South plays the ♥2 to the ♥A, East is squeezed when declarer next leads the ♣2 to the ace.
In this layout there is no way to get back to the hand by playing the ♣2 to the ace. In this case, the ♥A must have been cashed already at an earlier stage to squeeze East when declarer leads the ♣A and discards the ♦2 from the table.