Standox pretty much covered it but i think the crucial differance between regular topping and fimming is that with fimming you come away with 3+ top buds where as with topping generally you only get 2 top buds.
The way you fim, just to clarify, is to take off only a small clipping leaving a miny upside down pyramid shape to the top bud which should cause multi branching from that point, creating many new heads.
Hope this has been useful
Panda