There are some asking what is “bump firing” on an AR15. So here’s the quick and dirty sort of lengthy explanation on Wiki:
Bump firing is the act of using the recoil of a semi-automatic firearm to fire shots in rapid succession, which simulates the feeling of a fully automatic firearm. This 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.
This technique is usually used for entertainment, as the drawback of decreased accuracy eliminates any conceivable “tactical” advantage that might be gained. However when used in close proximity, the desired effect of many bullets hitting a target can easily be attained.
Now that you have an understanding of bump firing, see it in action here with Youtuber gal Lisa Jean.
Sources: Wikipedia, Lisa Jean Youtube, USAtacticalAmmo.com