I disengage (thumb stays on) after the muzzle clears the holster, but is pointed low and downrange on the way up... well before my support hand comes on.
Trigger finger stays along the frame/trigger guard until both hands get full extension to the target.
I prematurely launched a few embarrassingly poor initial hits when I reached full extension by punching out aggressively with my trigger finger in place. BANG ?
Hit the IPSC target, but not exactly where I wanted!
Don't believe that would happen these days as I'm much smoother, but it is already deeply ingrained to keep off the trigger and place my finger deliberately.