Ok I'm still learning, and since I'm not finding clear answers on the Internet, I figure I better get this info up, so others may benefit from my mistakes
FIM is performed in order to limit plant height and encourage lateral branching. This technique and others like it (SOG, SCROG, FIM, LST, Topping), are especially beneficial, when growing under artificial lighting, which has very limited penetration/capability. In other words it's good to keep your plant growth within the penetration area of your lamp. I believe FIMMing is preferable to topping, so reasons I'll go into later, but first a chart which shows the optimal penetration range for most HID lamps.
I've seen other tutorials on FIM, and they all seem to be somewhat confusing, or downright inaccurate.
FIMMing isn't rocket science, it's pruning the growing tip of a plant - just as topping is - but do it wrong, and you'll soon know why they called it FIM (F*ck I Missed)
For clarification: Topping is the removal of the growing tip at or above the 2nd or 3rd visible node, whereas FIMMing is the removal of the uppermost growing tip above the uppermost visible node - this growing tip is only visible, and accessible, if you part the tender new fan leaves. Clean fingers or a tweezers works well for this.
Note: When doing plant surgery always start with clean hands, and sanitized equipment.
First let's look at what happens when you FIM incorrectly. When removing the top 80% of the uppermost growing tip - as I've seen in some FAQ tutorials - you are likely to miss the growing tip and end up trimming your tender new fan leaves instead, and it's never a good idea to trim fan leaves! You can think of fan leaves as the plants solar panels; this is where the energy comes in, and the more energy in, the more bud out, hopefully.
In this first pic it's still unclear as to whether, or not, the growing tip was pruned out, or not, but what is clear is the fan leaves were pruned, and this is what you want to avoid - for obvious reasons.
In this second Pic you can clearly see the results of a failed FIM attempt, which, again, resulted in clipped fan leaves, and a growing tip that continued to grow. This plant was later re-FIMMed (properly), and LSTed.
The next Pic is the same plant in the second pic - properly FIMMEd and LSTed. As you can see this plant already has lots of branching, which will become the new tops. I'll try to get a better closeup of a successful FIMMing and have it up shortly... the tiny growing tip was removed, while leaving the precious fan leaves intact, and that's what constitutes a proper FIMMing.
This is a good graphical image, but again,
where it says "FIMM cut location" is NOT where you want to make the cut! In reality you need to gently spread the "new set of fan leaves" out of the way, find the growing tip, and then make the cut. Only then can you be sure you are pruning out the growing tip, while at the same time preserving the surrounding growth.
So lets recap:
1. Cleanliness
2. separate the new growth (fan leaves) using your fingers, tweezers, or whatever works for you.
3. locate the growing tip - that would be the main stalk - ONLY- where it says to cut, in the above pic.
4. Now comes the FIM: Carefully remove the growing tip, while being careful not to remove the surrounding growth. Be especially carefully if attempting this with a razor blade, because they have a way of cutting through more than you intended. RUI's MBlaze - whom I have much respect for - FIMMs his plants, by pinching out the growing tip, with his fingernails, and he gets good results, but I wasn't able to grasp his technique - in detail - from his tutorial.
I was able to find this pic, and doctor it, but again it shows the
cut being made across the new fan leaves (wrong). Of course, in order to make the correct, more precise cut, you're going to need a smaller scissors, but this shows the growing tip we want to cut.
To be continued...