White has firepower directed at the Black king’s territory; the question is how best to take advantage of it. What prevents White’s bishop and rook from converging on the square next to the king? The pawn on b7. Or you might see that White has no checks and that his only capture of a piece is BxB. Or you might have seen that Black’s rook is loose, and that Black’s king and rook form the kernel of a back-rank skewer that White can exploit if a clear path can be manufactured for his rook to use to reach b8. One way or another you are led to consider BxB for White. If Black recaptures b7xB, White now has a new check to consider: Rb8+—a skewer, since after Black moves his king White plays RxR. It's a little study in clearing the path to the skewering square.