The apex of the main stem, to top you cut the stem clean severing the xylem and phloem channels. There is no surprise, plants redistributed energy to side stems.
Inside the apex of the crown there are mini versions of future growth all in smaller and smaller forms at the very peak of the apex like layers of a onion skin, if you cut precisely you can cut a fudge I missed 2,3,4 crowns deep, beauty of a FIM is the results are very random and can generate anywhere from 2,3,4 possibly 6 or 7 tops depending how the cut is made.
Do a little Google for Mandelbrot set and watch a video, will give an idea of the infinity held within all life.
Tis very hit or miss, I've done maybe 6 or 7 fims and I'd be no more confident in achieving it than the first time i tried its very random. Bright side it doesn't screw with apical dominance the same way as chopping the apex clean off either.