The term "bump fire" was originally an improvised technique to shoot an AR-15 faster by having the shooter applying a non-rigid forward push on the receiver (by gripping the handguard or via a foregrip) and having a loose hold on the pistol grip. When the gun shoots, the recoil shifts the receiver backwards, moving the trigger conversely forward (from the receiver's frame of reference) and relaxes the pulling force on the trigger, allowing it to reset. When the shooter's forward push overcomes the recoil momentum and shifts the receiver back towards the front, the trigger is "bumped" against the shooter's finger and gets depressed again, firing off another round, which produces another recoil that repeats the above process. This allows a cycling rate of firing much faster than what the shooter's own finger can typically achieve, but is usually inaccurate due to the shooter often having to fire from the hip to still hold the gun firmly.
