Figure[White to move]

On the e-file we find a classic relative pin of Black’s bishop by White’s rook. The pinned piece is attacked once and defended once; to get mileage from the pin White needs to attack the bishop again. The usual technique would be to advance a pawn toward the target with f2-f4. Any problem with that here? Yes, for ask whether either of the enemy pieces in the pin would be able to leave their squares with threats—especially checks. The key observation here is that once White moves his pawn off of f2, he opens a line to his king. Now Black would be able to play Bd4, giving check. White has to move his king, which not only buys time for Black to move his pieces; it enables him to play RxR, winning a piece.

As we see here, an advantage of breaking a relative pin by moving the pinned piece in a time-consuming manner is that sometimes the screened piece then is able to take the offensive against the piece that had been inflicting the pin.