Instant Cluster Explosion
From Worms Knowledge Base
This glitch is only present in Worms 2.
If you stand on an opponent worm's head, aim in any direction and tap the spacebar, the Cluster Bomb will sit on the worm's head. If you jump upwards just before the timer finishes and the bomb detonates, all of the clusters will immediately explode, causing very large damage to the worm.
The same effect can be achieved by placing the Cluster Bomb under the worm when the worm is on a steep slope, but this is not a bug as such.