Black has one possible capture: BxN. The knight is guarded by White’s rook at b1. Black has no way to do anything to the rook; and the natural method of interference, Rd1, leaves his own rook unprotected and just results in RxR. But Black is methodical in examining any checks he can give and their consequences, and finds that there is one, Rd2. White’s only legal reply is Kc1—and now his knight is left loose because his own king blocks the path of its guard along the first rank.

The position shows a pattern worth watching for: sometimes a sequence you plan may have the surprising side effect of causing one piece to block the line of protection leading to another. The idea is worth special mention because it often catches its victim by surprise, too.