Unblocking play required when a menace is blocked and entries are lacking.
You temporarily establish a winner for an opponent, only to force him to discard it.
James Clay of London, a leading whist authority in the last century, first described the play. He ascribed its discovery to the best whist player in Vienna, hence its name.
In the end position on the following hand, North’s ace of hearts blocks the heart menace. To squeeze East you must cash the ♥A before crossing to the♦A.