The Lasker Trap underpromotion — Chess Library

Black’s pawn can promote while capturing the rook on g1. A queen would be worth more, but only a knight arrives with the check needed to make the combination work.

Lesson

Black to move. Main line: 7... fxg1=N+ 8. Rxg1 Bg4+

7...fxg1=N+! promotes with check. White must spend the next move capturing the new knight instead of organizing the exposed king.

After 8.Rxg1, 8...Bg4+ develops with another check and keeps Black’s attack moving. A queen promotion would not provide the same forcing tempo.

The best promotion is the piece whose movement solves the position. Usually that is a queen; here the knight’s unique check is the entire point.