In bridge, a trump promotion is any defensive play that "promotes" a trump loser into a winner. This is commonly done by forcing declarer to ruff a trick with a trump honor. When a defender ruffs a trick to force declarer to overruff with an honor, this is a particular type of trump promotion called an uppercut
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South declares 4 and appears to have two spade losers and one club loser. However, look what happens if West's opening lead is a spade. East cashes the top two spades and leads a third one. If declarer discards, then West will ruff. If declarer ruffs low, West will overruff. Finally, if declarer ruffs with a high heart, West's QJ holding is promoted into the setting trick.
A trump promotion can also occur when a defender has the opportunity to overruff declarer (or dummy), but chooses not to. This is the case in the following deal:
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South is playing in 4. It looks like there are only two heart losers and one spade loser. But if West leads a heart, East can win the top two hearts and play back a third round. South may well ruff with the T. In that case, West can discard a club or diamond and thereby score two trump tricks later. If West makes the mistake of overruffing with the K, South can win any return and draw the remaining trumps to make the contract.