A windmill is a very special type of discovered attack that you can use to win lots of material.
Definition-wise: a windmill is a repetitive discovered attack on the King using the same piece, to force the King to shuffle back and forth helplessly.
There are many spectacular examples of the windmill, but the “See Saw” is by far the most common (term coined by GM Murray Chandler in Chess Tactics for Kids). You have a Rook on g7 (or g2 if you’re Black) and a Bishop on the long diagonal. Then, you move your Rook and take a piece to unleash a powerful discovered attack on the King. When the King timidly steps back to g8, you lather, rinse, and repeat.
The windmill exemplifies the mantra, “Rooks belong on the 7th rank.”
One spectacular combination that uses the windmill is the Petrosian Draw. In this case, you use the windmill to force a draw by perpetual check. It is called the Petrosian Draw because usually the player sacrifices a lot of material to reach that position – if they can’t win, they can force a draw.
Coming soon: A new series “Forcing the Draw” which covers stalemating tricks like the Petrosian Draw.
Here are some more famous examples for your viewing pleasure! A shout-out goes to YouTube Channel “Chess with Suren” for featuring these wonderful windmill ideas.