By discarding a card or cards either from declarer's hand or from dummy or both, declarer can stop them from transferring the lead between each other, usually to prevent a defensive ruff.
NW ES Consider this hand and auction with an opening lead of the eight of hearts.
However, if East plays the queen, South (declarer) must win with the king, or else his contract will be quickly defeated.
This promotes West's jack to the highest outstanding trump, guaranteeing a third defensive trick.
By this Scissors Coup, East can no longer gain the lead.