A few months back I attended a Ron Covell beginning/advanced sheetmetal weekend class. One of the things Ron discusses is basic hammer and dolly work, and removing dents; here is the process. Ron whacked this piece of metal with a pipe. The red circle at the marker is the first point of contact, whereas the rest of the highlighted area is what ensued after that initial contact. When working a large dent you want to work backwards, meaning first correct the last affected area.