FIM stands for "Fuck I missed" technique. This is usually performed when the vegging plants are at their 5th or 6th node. However, you can FIM as early as you want to encourage bushier growth and more budding spots.
Essentially there are the same, "Pruning" and "Topping", just two different commonly words used. It is also known to some as "Pinching" as well. In this page it will be referred to it as "Topping" Topping is done to Marijuana to keep the plants small and bushy, promote "branching", and increase the overall yields of the plants. With higher overall yields, a grower will successfully harvest many more buds, or floral clusters, and from smaller, bushier and more compact plants.
If Topping is not done to Marijuana there is a lot less yield per plant. To the indoor grower this can be a waste of their artificial lighting, and growing spaces potential. Marijuana with its own natural growth pattern, and without the benefit of topping, will have one main central "cola" bud, at peak flowering. Several other small branches will grow outwards, down its main stalk, with much smaller bud tops.
Marijuana's natural growth pattern is to grow upwards at its main stalk. From this main central stalk will begin to grow side branches. The side branches come out as tiny shoots with leaves, and usually there are a pair of them on opposite sides of the main central stalk.
When the seed leaves have long dyed off on the main stalk. The first true seed leaves commonly can or will, wilt, dry up, or dye off of the plant as well. Once healthy new vegetative growth begins the rate of growth can be very fast, with excellent lighting supplied.
As the new growth increases the light reaching the lower portions of the plants becomes less. Thus is is common to see first leafs wilting and dying etc. Growers that see leaves wilting or dying, etc, will opt to pulling them off of the plants.
The main central stalk is topped of just above the branches that are coming out below it. A pair of scissors or your fingers can be used. Once the topping is done you can remove the two upper fans leaves as well. This will aid in the light getting to the newer vegetative growth, and other shoot tips, down the main central stalk.
There are no rules to where you top your plant or how old it needs to be. As long as your plant has shoots protruding further down the main stalk it is able to be topped. When topped the growth of the plant will be concentrated towards the new, younger vegetative shoots.
Once you have topped your plant(s) the younger shoots will rapidly begin growing. With the removal of the main central stalk the lower branches grow more. With topping completed we keep the plants on their regular lighting and feeding schedules.
Now each new shoot tip will essentially grow as the main stalk did, however the growth is not concentrated to only one central stalk. So as each new shoot grows outward new shoots will grow from each one of them stalks as well.
Therefore topping can be done again, and again, and as each shoot becomes a growing tip with other shoots forming down its stalk, it is removed. By completing these topping or pruning tactics, a grower can achieve any desired height, or desired bushiness, they desire in their plants.