A rare endplay in which you can cause a seemingly certain losing trump trick to vanish. For this to work the defending hand with trump length must have only trumps left while his partner must have the lead and be able only to play suits in which both declarer and dummy are void.
When his partner’s lead is ruffed, the defender with the trump length faces the choice of underruffing or overruffing and being overruffed again.
With spades as trumps, East, on lead, plays a heart. You, South, ruff and subject West to a ‘smother play’ or coup. You will play dummy’s ace of trumps on this trick only if West overruffs with the king.