White pins Black’s rook with his queen. But it’s only a relative pin, so it has to be exploited with delicacy. The rook can leave its position, and safely leave Black’s queen en prise, if it can give a check in the process that requires White to spend a move playing something other than QxQ. This doesn’t seem like a worry on the board as it now looks, but the point has to be kept in mind at all times; it has a way of becoming important at unexpected moments. Here, for example, how should White take advantage of the pin? The rook doesn’t defend anything, so the usual idea would be to add attackers to the target until it comes down. White’s only available attacker is his own rook, so he might naturally consider playing Rc1. But now the point mentioned a moment ago has bite: Black then plays RxR+; and after White fends off the check with QxR, Black’s queen is free and the pin has resulted in no gain.

How can White avoid this problem? He must use his rook to attack Black’s in a way that does not allow RxR+. There are two ways to do this. He can play Rf2 now and then Rc2 a move later; or he can play Kf2 now and Rc1 a move later. The point either way is that when White’s rook attacks the pinned piece, it must not be on the same line with its king. Of course with either of these sequences Black’s rook still can leave its square with check (via Rc2 or Rc1, as the case may be), but that’s okay; White would be able to take the rook with his queen on either of those squares and win the piece. What you want to avoid is letting Black make a move that is a capture and a check, or that lets the rook check without White being able to capture it. (It’s true that if White plays Kf2 here he gives Black a move to regroup, but there is nothing useful he can do with the time.)