Fim is to not simply top but to damage the growth tip so you get
lots of tips not two.
Yep. Fimming is similar to topping, but you leave something like the bottom 10% of the growth tip, rather than removing it completely. FIM stands for "F*ck, I missed", since it looks like the result of a botched topping. As mentioned, it can lead to up to 8 growth tips, rather than the 2 you get with topping.
There is a technique to it, but it is hard to explain in text. Basically, you try to make a cup shaped cut, rather than straight, sort of scooping out most of the tip. Search online for a better description, preferably with pics.