The Reserve Tempo: a3 Wins Where a4 Draws

Watch the method: press play to see the winning idea run, or step through it move by move.

A pawn that has not yet been pushed to the front is not backward, it is loaded. Each step it still has left 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, only draws right now and loses outright once your king reaches a6.

Why the pawn was kept on a3. On a3 it still owns the step to a4, and once it sits on a4 that tempo is gone and the same picture is only a draw. After a4 Black must move, and every move he has walks into the outflank. Spend the step too early 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.