To promote a trump trick in a defender’s hand through leading a plain suit in which declarer and one defender is void. This occurs mainly in two ways:
1. The lead of a plain suit card through declarer can allow partner to make an extra trump trick.
2. If a defender ruffs with a high trump, this may force declarer to overruff, thereby promoting a trump trick for that defender’s partner.
Suppose the trump layout is as follows:
1. If East leads a suit of which both South and West are void, declarer faces an unpleasant choice: ruff low, allowing West to overruff or ruff high and lose the third round.
2. If West leads a suit of which both East and South are void, East should ruff high with the ten. If declarer discards, the defenders have an immediate trump trick. If declarer overruffs, West wins the third round.
As a rule, declarer does best to discard a loser rather than incur a trump promotion.
To thwart this, the defenders should usually arrange to cash their side winners before trying for a trump promotion.