Survey White’s possible captures and you find QxN and RxR. Of course both targets are protected, so turn your attention to their guardians. See the vulnerability in the Black queen’s protection of the rook on h4: White’s pawn on f5 can step between them, and with protection. The move attacks Black’s queen, so he has to do something about it. If he plays Qxf6, White has QxQ+; Black then takes White’s queen with his g4 knight, and White follows by at last playing RxR+ on the h-file—the point all along. He has traded queens and won a rook for a pawn. If Black instead replies to f5-f6 by playing Nxf6, he blocks his rook's line of protection and so again lets it go in trade for the pawn.
Part of what makes interceptions of this sort dicey is that, like the threats against the guard examined in the previous chapter, they often give your opponent wide latitude in choosing a reply. Here White’s pawn push f5-f6 threatens Black’s queen, but in addition to the moves just considered Black also can reply by taking the offensive with check: RxR+, with the idea of removing his queen from danger after White takes his king out of check. It is important to see this idea—and not to be dissuaded by it. For after RxR+, White has the decisive reply QxR+ (never back down from considering every check); and now White will mate a move later, as his pawn on f6 seals off g7 as a flight square for Black's king.
The defensive idea illustrated by this last variation still is worth pondering: if an "interfering" move cuts off protection to a target and makes another threat, one line of response is to pick up the target and make trouble with it. Perhaps the trouble easily can be evaded, but its creation still buys time to then extinguish the second threat, too.