Zwischenzug (German for "Intermediate Move") occurs when you delay an expected move—usually a recapture—to play a more forcing move first. This "in-between" move often changes the result of the game from a draw to a win, or a loss to a save.
Black (Ponomariov) just played ...Qxg5, capturing White's Queen.
99% of players would instantly play hxg5 to recapture. But Vishy Anand saw something deeper.
34. Rf7+!
Anand inserts an Intermediate Check. Black must respond to the check. This forces the Black King to a worse square. Only after the King moves does Anand recapture the Queen, winning a crushing endgame.
Anand vs. Ponomariov, 2005The concept isn't new. The greatest legends of chess history—Morphy, Capablanca, and the romantics—used the Zwischenzug to devastating effect.
Zwischenzug (German for "intermediate move") is a tactic where, instead of playing the expected move (usually a recapture), a player first makes a forcing move (like a check or mate threat) that the opponent must answer.
It is a specific type of Zwischenzug where the intermediate move is a check. This is often the most powerful type because the opponent has no choice but to respond to the check immediately.
The concept is as old as chess, but the term was popularized in chess literature by Fred Reinfeld and Irving Chernev in 1933.