What keeps the carabiner from coming off before you are on the ground?
In order for the Firefly to operate the climber's weight needs to be off the climbing rope. This allows the quickdraw to lift off the hangar -- if the climber's weight is still on the quickdraw the system is unable to lift up and off.
How does it work?
The Firefly uses a patent-pending system to open the gate of a carabiner and lift the carabiner up and off a fixed point.