So you're saying that the solvent weld connection is something that softens at a lower temperature than the bulk PVC? Or does the technique depend on a temperature gradient, that the pipe to be removed is hotter than the fitting that is to remain?
Do you know if it also works for ABS and ABS cement?
Cheers, Wayne
Calling a conventional PVC-to-PVC solvent bond a "solvent weld" isn't quite accurate. Solvent diffusion inside a polymer is complex (this is my area of academic expertise). The joint, for reasons of geometry but primarily of the material itself, isn't what you'd expect by saying it's "welded"; i.e., the polymer chains, even somewhat solvated (that's a long and complex discussion in and of itself) don't significantly interdiffuse.
In any case, you're really softening the inside piece of PVC. Of course, because of heat transfer, the outside will soften too, but an (almost completely) amorphous polymer like PVC doesn't melt like an inorganic material like solder or something. It softens rather gradually. When you soften the inner pipe of the bond, you can easily deform and fracture the adhesive polymer that bonds the joint together when you peel it away.
You're right to be skeptical. I was, too, even with my background. But I applied the technique twice in my basement where the consequences of screwing up were severe (breaking concrete) and both times it worked absolutely perfectly. If you're intrigued (it will fix your problem and do it with tools you have sitting there right now and cost you nothing), try it out on a couple of scraps - a cheap coupler and a piece of scrap pipe.
I would do it again in a heartbeat, and to the point of recommending it to folks like yourself. Just take your time and heat relatively slowly - don't burn it. And oh, to the comment below, PVC really isn't that flammable (over 50% by weight of the material is a halogen (chlorine)), so the chances of you setting something on fire is pretty damned low. ABS burns readily. You won't be holding the flame on it for much time at all, nowhere near what it would take to ignite it. And if you are scared of the flame, just get a cheap heat gun (I don't think a hair dryer would be suitable at all, though).