In chess, a windmill is a tactic in which a combination of discovered checks and regular checks, usually by a rook and a bishop, can win massive amounts of material. This tactic is also sometimes referred to as a see-saw.
In the game pictured at right, Carlos Torre-Repetto makes use of the windmill tactic against Emanuel Lasker to win two pawns and a bishop and enter into a winning endgame (although the bishop had to be given back). The move 25. Bf6!, hanging the queen, sets up the windmill. Black must accept the sacrifice, as his own queen is unprotected, and any attempt to stop the windmill would simply give White the queen. Then 25. ... Qxh5 26. Rxg7+ Kh8 27. Rxf7+ Discovered check, by the bishop. White simply repeats the regular check/discovered check pattern, taking as many pieces as he can with his rook. 27. ...Kg8 28. Rg7+ Kh8 29. Rxb7+ Kg8 30. Rg7+ Kh8 31. Rg5+ Kh7 32. Rxh5 White concludes the windmill by taking the black queen.