I agree 100% with what shubui says above, however, I use a different technique.... because I am after a
different end result.
You have to match your technique to your aim.
I know from previous discussions that shibui is developing nicely shaped trunks of good proportions, with good movement, taper, and a fair nebari.
He will end up with some really nice trees.
However, It is my opinion (and it is only my opinion and personal taste - it doesn't make me right or wrong), that a nicely developed trunk of good proportions is much prettier to the eye, and a much classier finished tree, in a
Japanese maple, rather than a trident... so I use shibui's method for my Japanese maples.
My aim with tridents is all about nebari and taper (sumo). IMHO, this is what tridents lend themselves to (as well as ROR). Any other form I'd rather see in a Japanese maple.
So, I prune my field grown tridents once per year in August. This includes trunk chops when necessary, as well as side branches when necessary.
Importantly, it also includes root work
every year - you need to control the lateral growth of the nebari carefully. To neglect this is to neglect the point of my finished tree. I also use this opportunity every year to thread seedlings into the nebari to both fill gaps, and expedite the lateral spread.
This year, shibui supplied some tremendous seed for this purpose.
I then let them grow wild for the full growing season and feed them profusely, once per week for the full growing season too. I can easily get 3-4m of height in a season. Then rinse and repeat the next year.
Scarring seems to heal fairly quickly on tridents, so this is not a major concern for me - and scars seem to heal especially quickly during the field growing process.