How a ladder works
A ladder (shichō) is a sequence of repeated ataris. You atari a group; it can only run by extending; after extending it has two liberties; you atari again from the correct side, back to one; it extends again — and so on in a zig-zag all the way to the board edge, where it runs out of room and is captured. Before starting, read whether the ladder 'works': a white stone on the escape path breaks it, while a friendly black stone ahead — like the marked one here — keeps the chase tight. If it doesn't work, attempting it loses sente and gives the opponent thickness.