Don't play useless moves
White's group is a ring with a two-space eye inside — alive-looking from a distance, dead-soft at its centre. The vital point of any two-space eye is inside it: a stone played there sits on one liberty, threatens the eye, and forces White to spend the whole space answering. The typical wasted attack is peeping or pushing from outside, moves that threaten nothing and hand White free forcing stones — always ask of a move, what does it threaten? Black to play: play the point that reduces the eye to one.