When is an eye not an eye?
An eye is real only if your stones control most of its diagonal points; the enemy filling the last diagonal turns it false, because then the two halves around the eye can never rejoin without filling their own eye first. Here White has wrapped the marked point into a perfect eye shape — but Black already owns its lower-left diagonal, so the empty one decides everything. A group with one real eye and one false eye still has only one eye, and one-eyed groups die: eye counting, not stone counting, is the law. The false eye is the quietest way to lose a group — no capture needed, just one diagonal at the right moment. Black to play: take the empty diagonal.