Backwash squeeze is a rare squeeze which involves squeezing an opponent which lies behind declarer's menace. A variation of this, known as the "Sydney Squeeze" or "Seres Squeeze", was discovered in play at a rubber bridge game in Sydney, Australia, in 1965, by the Australian great Tim Seres; it was later attested by famous bridge theorist Géza Ottlik in an article in The Bridge World in 1974, as well as in his famous book Adventures in Card Play, co-authored with Hugh Kelsey.
By nature, backwash squeeze is a non-material trump squeeze without the count. It occurs when the declarer (or dummy) has high trump(s) but must not draw opponent's remaining trump(s). Instead, he ruffs a card high, and the opponent playing after, still having trump(s), must choose to under-ruff or give up one of menaces, either in form of a direct trick or an exit card, allowing later endplay. Since the squeeze may be without the count, the squeezed defender might take a later trick.