The Roller-Delayed Contact Breech is a mechanism designed by Hurricane Arms to reduce the rate of fire on air guns.
The basic principle is that the breech is closed with a round in the barrel. Air flows into the barrel from the valve pushing the BB/Projectile until it imparts on a tiny sping loaded pin that sits in the barrel. As the BB pushes the pin, the breech it's attached to slides forward and opens. The pin then moves up on a triangular cam ramp and out of the way allowing the BB to exit the barrel.
Essentially the kinetic energy of the first BB in the chamber initiates the cycle for slower automatic fire.