FIM stands for ''Fuck I Missed'' and im not sure how it got that name because its very simple to do. I use this method all the time to help create a wide canopy with many branches and colas. Some people FIM each plant more than once to create even more colas and branches and it works very well for them and has very minimal stress on the plant.
I usually FIM plants when they are between 1-2ft tall and it should be done at least 2 weeks before turning to 12/12 to maximise the growth potential of the new shoots and maximise the benefits of using this method.
#1 Locate the very top of the new growth.
#2 With a clean, sterilized scissors, Fold the fan leafs over and cut approximately 80% of the new growth off the plant.
#3 This is what it should look like after the cut has been made.
#4 View of the Cut section after 2 days growth, showing the 4 new growth shoots (branches)
I sometimes dont even use scissors and I just pinch out some of the new growth with my fingers. Heres a few pics of another plant that I pinched with my fingers. This plant has been neglected throughout its short life so far and it has been sitting under a shitty fluro light so it would be doing much better under some decent lighting and better conditions but it gives you an idea of the early progress.
You can see the dot where the FIM was done.