Here's a still more spectacular and demanding illustration of our current principles. Notice all the firepower Black has trained on the White king’s position: bishops aimed at it in criss-cross fashion, though one of the bishops has its path blocked by the knight on d4—a knight which, however, is poised to give check from c2. The hitch is that c2 is guarded by the bishop on e4. An idea comes into view: imagine capturing the e4 bishop with 1. …QxB+ and ask what happens next. Naturally you worry about the recapture NxQ; but you don’t worry too much, because then Nxc2 is mate—a mate of the invisible smothered variety, as White’s king appears to have plenty of open flight squares but in fact has none that are safe.
White has other options but they, too, all end in mate. Thus 1. …QxB+; 2. Kf2, Nf5+ (discovering check by the bishop and covering g3); 3. Be3 (interposing), QxB#. Or 2. Be3 (interposing), Nxc2+; 3. Kf2, QxBe3#. These possibilities are worth some study, as they again require the ability to visualize how the king’s movements are constrained by invisible lines of attack.