Promotion

Promotion is the tactic where a pawn on the seventh becomes a queen, or where the threat to do so decides everything else. A pawn on the seventh costs a piece to stop; a pawn on the seventh whose queening square you also attack costs a piece to stop and gets nothing back. All the tactics are about the last square: who controls it, who blocks it, and how to change that.

What makes it work is that the blockader is a piece with a job. Deflect it, with a check it must answer or a capture it must recapture, and the pawn goes through. Or clear the queening square with a sacrifice the defender must take with the wrong piece. And promote with check, or with mate, whenever you can: a new queen that gives check is a new queen nobody takes.

To spot it, every time you have a pawn on the sixth or seventh, look at the queening square and list the pieces that cover it. One defender means one deflection. When two pawns are running, count the race: who queens first, and does the first queen control the other pawn's square?

At higher level: the underpromotion, a knight that promotes with check or forks, a rook that avoids stalemate; the promotion by capture that changes file at the last moment; and the sacrifice of the queen you already have to get a new one, because the new one arrives with tempo and the old one was tied down.