The Belladonna coup is the play of a low card away from an accompanying high card, giving the opponents the impossible choice between setting up a winner for declarer and abandoning an attack on another suit.
[2] Both sources attribute the coup to Giorgio Belladonna, for many years a cornerstone of the Italian Blue Team.
The best prospect is to ruff a heart in dummy, but the attack on trumps jeopardizes that plan.
If South mis-times the play, the defense can manage to lead three rounds of trumps and win the ♠K, before declarer can ruff the third heart.
This gave the E-W an impossible choice: Notice that South gives up the best chance of making a heart trick (leading toward the ♥K).