The Kieninger trap is lesson 6 of 10 in Brilliant sacrifices 1: classic traps, an opening course suited to players rated around 900 (beginners). Budapest Gambit, A52. Black leaves a bishop hanging, and a knight gives smothered mate. It has 4 interactive exercises: 4 find-the-move puzzles. You play every move on a board and get feedback straight away. The lesson is free.
What this lesson explains
This is the Budapest Gambit. Your bishop on b4 pins the knight on d2. White's a3 attacks the bishop. Look at d3 instead. About 181,000 rated games on Lichess have reached this position.
Ngxe5! You win back the gambit pawn and leave the bishop. If axb4, a knight jump to d3 is mate.
After axb4, Nd3 is mate. The pawn on e2 cannot take, because your queen pins it to the king. The king is boxed in by its own pieces.
Recap.
In the Budapest, Bb4 check pins the knight on d2.
If a3, ignore it: Ngxe5 wins back the pawn.
axb4 runs into Nd3 smothered mate.
Your queen on e7 pins the e2 pawn.
What you practise
First play the line. You are Black: play the moves of the Budapest Gambit, up to the moment of the sacrifice.
Your turn: find the sacrifice. You are Black. White attacks your bishop with a3. Do not move it. Win back your pawn with a knight.
White sees the mate and takes your knight instead. Take back, and keep the pressure.
This time White plays e3, not a3. Win back your pawn now.
Worked examples
First play the line. You are Black: play the moves of the Budapest Gambit, up to the moment of the sacrifice. Good. White answers a3. Now the position is ready for the sacrifice.
Your turn: find the sacrifice. You are Black. White attacks your bishop with a3. Do not move it. Win back your pawn with a knight. That is the Kieninger trap. If axb4, Nd3 is smothered mate. The pinned e2 pawn cannot take.
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