Two blocks, one direction
White's three edge stones are weak and must run — and you choose which way. The profitable attack blocks the open side, so White crawls along the edge toward your corner wall: every extension it makes stays small, while each of your chasing stones builds the outside. The wrong block pushes White away from your thickness into open board, where it settles comfortably and your attack earns nothing. Your wall sits at the lower left; the contact play that matters is at the other end of White's line.