That is an old school method of dent removal. Heat is almost never used on dent work unless the bell has been totally crumpled and folded over on itself. Annealing a dent in the course of normal dent work is extremely rare because it adds much more cosmetic damage and cleanup. It also just isn't necessary. Annealing a dented area would only be used in the most extreme bell damage, when the brass is crumpled and may have hardened to the point where the brass will crack upon reshaping the creases. Even so, this would only be used in the case of full mechanical and cosmetic overhauls. I have never had to anneal when repairing, and the other techs I have talked to about similar things have just about never had to either.