A wire gets longer when you heat it. Now bend this wire into a circle... the wire gets longer, so the circle expands. Now imagine the pulley as a bunch of concentric circles of different diameters... each one expands and grows in diameter.
I bought an HF Earthquake gun today after having trouble with a crank pulley bolt (Acura Integra). Still couldn't get the bolt off... perhaps I should have tried a different and/or shorter hose.
Ended up putting the trans in 5th gear, having a friend step on the brakes, and using a breaker bar...