It is a trigger group that sets when you pull the trigger and fires when you release the trigger. Fairly common among ATA trapshooters struggling with a flinch.
Operation.
Load one round,
shoulder gun,
pull trigger, CLICK!,
call Pull!
track and lead the target, release trigger, BANG!
If you get a broken target or no target, hold back on the trigger, reach over with your other hand to the action release on a repeater or the top lever on a break action, get the action open, then let off on the trigger and start over.
An over/under for doubles shooting may be set up as either release-pull or double release.
I once saw a Model 12 trigger group that had been set up to work either way. The crossbolt safety button became a selector between pull and release trigger operation.
Some release triggers were subject to "pulling through." If you pulled the trigger too hard to set it, it might well fire right then. That is the source of a lot of those patterns on the back wall of trap houses.