One Reserve Tempo Wins the Opposition

A pawn standing on its original square is not backward, it is loaded. It can step one square or two, and that choice is a tempo you can spend exactly when the zugzwang needs it. Steinitz built a whole rule on this and it decides the present ending.

Three only-moves in a row. Ka6, then a4, then Ka7. Every one of them is the single move that keeps the win. In particular c5, the natural pawn break, loses outright, and a5 (two squares instead of one) only draws.

Why a4 and not a5. After a4 Black must move, and every move he has walks into the outflank. After a5 the a-pawn has spent both of its steps at once and it is White who runs out of waiting moves.

The companion is a draw. Our position "The Opposition Is Worthless Without an Outflank" is this same picture with the a-pawn already sitting on a4. The reserve tempo is gone, and with it the win.