The bump firing process involves bracing the rifle with the non-trigger hand, releasing the grip on the firing hand (leaving the trigger finger in its normal position in front of the trigger), pushing the rifle forward in order to apply pressure on the trigger from the finger, and keeping the trigger finger stationary. During a shot, the firearm will recoil ("bump" back) and the trigger will reset as it normally does; then, the non-trigger hand pulls the firearm away from the body and back to the original position, pressing the trigger against the stationary finger again, thereby firing another round when the trigger is pushed back.
Normally, a rifle is held securely and firmly against the shoulder, but the loose shoulder hold that creates the rattle to rapidly depress the trigger affects accuracy in a way that is not encountered with firearms that are designed for select-fire.
Bump fire stocks are gunstocks that are specially designed to make bumpfiring easier, which allows semi-automatic firearm to mimic the firing speed of fully automatic firearms.[1] They can achieve rates of fire between 400 to 800 rounds per minute.[2] As of 2017 bump fire stocks in the United States sell for about $100 and up.[2]
https://en.wikipedia.org/wiki/Bump_fire