From Worms Knowledge Base
This is the complement of Mitosis. It enables a worm to fuse/merge with one or more members of its own team, into one worm. The new worm possesses the sum total health points of the worms that fused, and it bears the name of the worm that activated the fusion.
The new worm is just a regular worm in appearance, so the only new graphics required is the animation of the fusion.
Even though it's probably better to have two weak worms than one healthy one, there can be situations where a fusion would be helpful. For example, to save a weak worm from being easily killed, or to alter the turn sequence in the absence of worm-selects.