FIM is a different techinique.. It is topping, but UB tops large, where as FIM is topping small.. Actually, if done properly (which is easier said than done unless you've got the hands of a brain sugeon) FIMing is actually precisely topping before the topmost two nodes stretch apart.. The node you see, and the infant node inside of it, before it grows further upward..
Hormone shifts are identical, with FIM you may still get new shoots coming out at lower nodes, but as UB stated he takes the plant down to the second node because topping a plant pretty much will always facilitate strong new growth for 2 nodes downwards, and anything further is unpredictable (unless you know from experience what the particular plant responds like)..
Anyways, so when you FIM properly, you remove the apical meristem, which stops the nodes from separating, and your two new outputs from each of those nodes will grow upwards from virtually the same place instead of being separated by one normal length nodal spacing as UB's method does.. If you FIM improperly, you'll damage the infant node, and either won't get outputs from it at all, (if this occurs you will likely get outputs at the node directly below the visible node you cut at, since as stated most plants are good for shoots from atleast 2 nodes or you'll get failed outputs from the infant node, two outputs from the visible node, and possibly more below..