The blowtorch is a tried and true method for removing pesky bolts, and the scientific principle behind it is the expansion of metal when heated, as paradoxical as that may sound.

Some mechanics recommend heating only the nut (or whatever the bolt is screwed into), so it will expand but the bolt won't. The bolt will then turn more easily in a larger socket. (A metal nut doesn't expand outward in all directions like a baking doughnut, with the hole getting smaller as the outside gets bigger. Instead, the diameters of both the nut and the "hole" expand, making the nut "looser.")

This explanation makes sense, but as you and many others have noticed, heating the whole assembly at once — or even just the bolt — usually works just as well, so there has to be another explanation.

There are a couple of factors at work here:

1. The act of expansion is also an act of motion. While the bolt and the nut are expanding, their surfaces are rubbing against each other, and the friction between them is often enough to break up the rust or crud or whatever is sticking the two pieces together.

2. The bolt and nut may expand at different rates, especially if they're made from different materials. This accelerates the effect mentioned above, and may also be enough to loosen the two pieces if the nut expands faster than the bolt. This can also happen, in the opposite direction, during the cooling stage.

Source: Renovation Headquarters, The BugShop, Halfbakery, DIYprojects.info