And now one of the most exquisite of all tactical motifs: the windmill. A windmill generally involves a series of discovered checks in which the masked piece and the unmasking piece take turns checking the enemy king; the masking piece, on its “off” turns, captures one enemy piece after another. (This will become clearer in a minute.) Although a windmill can in principle occur with other pieces, the ones you are most likely to see involve a rook discovering check by a bishop; that is why the motif is included in this chapter. These also typically are cases where the kernel of the discovery does not exist at the outset of the sequence. It has to be built, and in the building of it the enemy king is forced into position for a discovered check. This, too, will be easier to grasp once it is seen.
Begin with the simple and modest use of the idea to the left. Black has the kernel of a discovered check: his rook masks his bishop’s path to White’s king. Black just needs a good target for his rook. White’s rook won’t do; for after Rf2+, White moves his king to g1, protecting the rook as well as evading the check. Black needs a target farther away. The bishop and pawn on the other side of the board protect each other; does it matter? No. Black plays Rxa2+, taking the pawn; White is required to play Kg1 (the king's only flight square); and now Black brings the rook back to g2 with check. Now the key point: White has to move his king back to h1, and suddenly the position has been reset to the beginning with Black ready this time to go after a fresh target—White’s bishop, now left unprotected. Black plays Rb2+, and after White moves his king back to g1, Black plays RxB. A simple instance of a windmill.