The cut
Amount
Trim about 80 percent of the apical tip and leave about 20 percent intact
Not a full top
A clean topping usually makes two to four new leaders. A fimmed stub can make six to eight
Tissue
You are leaving a fragment of the growing point, not stripping the node bare
Clean
Use a sharp blade so the stub is not crushed
What you get
Bush
Multiple tops on one branch fill a canopy faster than a single spear
Weight
More heavy branches, if you then support them
Repeat
The book describes this as a veg tool for a plant laden with tops, not a weekly haircut
Support
Those extra colas will need stakes or a net once they flower
How it differs from topping
Topping
Removes the tip entirely and releases the pair of side shoots
Fimming
Leaves a remnant so more than that pair can run
Height
Both slow the main spear. Fimming aims at more sites, not just two
Choose
Use topping for a simple two-by-two. Use fimming when you want a cluster from one limb
Aftercare
Light
Keep the new cluster in the canopy sheet so weak inner shoots do not stall
Air
A sudden thicket of tops needs airflow or humidity will sit in the middle
Feed
A plant that just multiplied its tips still wants the veg nitrogen program until flip
Flip timing
Let the new tops set before you change photoperiod, or you will flower a mess of uneven spears