FIM-ing refers to (or at least used to) a technique by which you top or prune a plant to get more than the usual 2 branches that usually grow from the cut.
I've never done it, but from what I understand the cut is made at
a node rather than between one.
It's fallen out of favor when people began to realize like Hick said, that more branches do not mean more yield. It means more but smaller buds.
I heard it was mainly used nowadays for scrog grows.
There is a way to get more yield from more branches. You need to supply lots of soil (for a bigger root mass), more nutrients and more light.
Outdoors, in the ground where there is usually more soil and plenty of sunlight, multiple toppings can up a yield dramatically. The biggest yield I've ever seen from a single plant--over 3.5 lbs.--came off a heavily topped plant that was only 5' tall but about 8' across.
The pot was crummy though, which might have been from trying to get the plant to do something it was unable to do--produce enough THC for all those buds.