This part protects the gear and shaft of your Parrot AR.Drone 2.0 when it hits the ground.
I simply put 3 dots of cyanoacrylate glue to stick it on the motor support. You need 4 gear and shaft protections per AR.Drone 2.0.
Needless to say that it helps to protect your shaft and gear, but I can't warranty 100% efficiency. Nevertheless I've drastically increase my gear and shaft lifetime :-)