Zwischenzug is a German word that means an in-between move. The idea behind a zwischenzug is straightforward. Imagine you are considering initiating a sequence of exchanges that you expect to go something like this: "I capture, he captures, I capture, he captures, and then I win a pawn." Sounds good, doesn't it? So you proceed with your idea, and it comes out like this: "I capture, he captures, I capture, he plays check? OK, no problem. I simply move my King. Oh, oh. Now he captures with a check! Oops." What happened? You are the victim of a zwischenzug. Your opponent sneaked in a move before the anticipated recapture. Lawyers say you should never ask a witness a question if you don't already know the answer. Well, zwischenzugs are the unexpected retorts of chess. Usually (but not always), zwischenzugs are checks.
Diagram shows a zwischenzug in its most basic form. Black is very happy with his position. A pawn ahead, he is anxious to trade pieces and push his a-pawn toward coronation. Overlooking a zwischenzug, Black confidently plays 1... Rxh4 expecting 2.Qxh4 f6, which leaves the a-pawn ready to speed down the board. Instead, White plays 2.Qd8+! Kh7 3.Qxh4+. The havoc wreaked by the zwischenzug is obvious. White has recaptured with check, and with the safety of his King at stake, Black doesn't have time for 2...f6. After 3... Kg8 4.Qd8+ Kh7, White can claim a perpetual check with 5.Qh4+. Alternatively. 5.Qxa5 snaps off the a-pawn.1...