I agree it looks very much like chop and grow trunk.
That's how I grow most of the trunks I sell here.
Tridents I can chop each winter but slower growing species need a few years between chops to grow.
First chop is aimed at around the height you want the first bend/ branch so will depend on the ultimate size you are planning. Typically my first cut is around 5-7cm (2'-3'). Subsequent chops are higher each time allowing each successive section to be thinner than the parts below.
Check some of the photos in the trident maple catalogu on Shibui Bonsai.com.au -
https://shibuibonsai.com.au/?page_id=215
Regular root pruning promotes better nebari. Better nebari causes the base of the trunk to swell out even wider at the roots so even if regular root pruning slows trunk growth (I'm not sure it really does) that's more than made up in the increased basal flare.
Regular trunk chops means the earlier chops are healed or partly healed by the time the trunk is ready for a pot which is a great time saver during the subsequent branch and apex development phase.