Again the pieces at issue all are protected but aren't equally secure. Black attacks White’s bishop with one of his rooks and he attacks White’s rook with the other. The bishop is protected by a pawn, and so cannot itself be taken with any gain. White’s rook is protected by—his bishop. So first Black takes the bishop with RxB. After White recaptures with his pawn, White’s rook is left to be taken with RxR. To put it more simply: you ask why you cannot play RxR, and see that the rook is protected by the bishop; so you capture the bishop.
Removing the guard requires you to think backwards. You notice something you might be able to take but see that it has protection. Instead of dismissing the idea as unworkable you ask how the piece is protected and whether you might change that. Thus you never stop with the conclusion that a piece is protected; you always finish the sentence: protected by what?