Well, to FIM, you have to cut off more than the leaves. You have to also cut through the apical meristem, just like topping. The difference is that the cut is through the developing node, not between nodes.
You could do a FIM at the first or second node, but the real benefit of both FIM and topping is the tremendous secondary stem growth that occurs below the cut.
I did this FIM through the seventh node, and removed secondary growth from the first and second nodes, leaving 4 strong pairs of stems below the fimmed node. This seems to produce explosive growth, with lots of tertiary stems popping up along them.