It's an "inertial starter drive", as the starter motor rapidly spins up, inertia holds back on the rotation of the gear, and causes it to screw towards the ring gear teeth, once it "catches" it's held stationary and finishes extending, at that point tuning the engine.
The big spring exists to "cushion the blow" when it grabs.
The worm threads on the drive need to be clean and dry for this to work.
As soon as the engine "catches", and drive gear speed exceeds starter speed, the process is reversed, and screws the drive gear "out" of mesh with the ring gear.
If it is a newer-style "positive engagement" drive, it is locked from kicking out until the starter and drive reach a much faster speed.