You put your finger inside the bottom ring, pull it backwards to start firing.
The cocking handle moves backwards after ignition, eject the spend shell and chamber a new one into battery.
As the handle moves forward, it will jerk the rope and where the rope circle the back of the trigger guard it creates a pulling action onto the trigger.
Your finger on the ring is now an achor point for the rope and thus, as the rope was pulled tight, the trigger is pulled back.
And so the actions repeats, until you relax your trigger, at which point the anchor of the rope is no longer there and the jerk of the rope is no longer transferred to the trigger.
Simple device but very very effective.
I saw them in used in Indonesia by some faction (freedom?) fighters during my younger and stupider days (not that I am old now compared to some old fart like bushy, vint or vince