In the electro fusion process a heating wire permanently molded into the fitting gets heated by current at a low voltage. The surrounding material around the wire gets melted and expands due to the warming. The melt starts closing the gap between pipe and fitting, leading to a heat transition to the pipe which also gets heated and melts in the welding zone. The pipe material also expands.
The increasing material expansion between pipe and fitting, which is hindered by the cold zones outside the welding zone, leads to a build up a welding pressure between pipe and fitting and a homogeneous welding connection emerges.