Here is what Bill Fretz has to say about Raising:    
Raising is the compression of metal down to the stake without stretching it. The trick is to angle the metal off the stake so that there is an air gap for the compression. The Wide Raising Hammer can be used to raise sheet metal into a bowl against a wooden stake or a metal t-stake. It is also useful for raising in cylinders to form a concave shape. The Wide Raising hammer also makes it possible to planish subtle concave shapes. The size of the hammer will be dictated by the size of the work. Small light projects will use the mini hammers and the larger work will demand the large hammers. The concepts of what they do are the same.

Save even more when you buy Fretz hammers in sets!