White's centre stone is not yet in danger, but its best escape direction leads toward your upper-left stone — and a net plays best with a friend behind it. The diagonal cap, the small knight's net (小飞罩), seals the route without touching a single liberty of the stone: no atari, no forcing reply, nothing for White to profit from. That looseness is the point — a chased stone dodges ataris and grows stronger, but a netted stone only watches its exits close. Black to play: net the runner, letting your upper-left stone back the play.