A forcing defense in contract bridge aims to force declarer to repeatedly ruff the defenders' leads. If this can be done often enough, declarer eventually runs out of trumps and may lose control of the hand. A forcing defense is therefore applicable only to contracts played in a trump suit.
The defense should try to make declarer ruff in the long trump hand. Unless declarer is playing for a dummy reversal, he usually intends to ruff losers in the short trump hand anyway. If the defense can shorten declarer's trumps sufficiently, it may wind up with more trumps than declarer. In that case, the defense will be able to pull any remaining trumps and run its own winners.
A forcing defense is usually begun on the opening lead because the tempo is often important. It is indicated when:
The latter requirement means that the forcing defense is seldom attempted against voluntarily bid slams: the declaring side normally has so much strength that the defense's opportunities to continue to attack the trump suit are very limited.