Ever wonder how to do it or just what in the heck everyone is talking about when you hear that phrase?
From FULL-AUTO.com, credit Rabid Coyote:
"Bump firing is a technique utilized by people who like to make noise and waste a lot of ammo.
Its an inertial firing technique. A rigid finger is placed in the trigger guard without the trigger hand really grasping the weapon. The off hand is used to pull the weapon forward, tripping the trigger against the stationary trigger finger. Constant forward pressure is applied with the weak hand, and recoil causes a pulsing movement of the rifle, tripping the trigger continuosly against the rigid and stationary trigger finger.
Some people can do this at a rate that appears to approach cyclic rate."
Then credited to Snipe17 for the following: (how-to)
"Essentially, you hold the gun about waist level. Let it slide loosely in your right hand with trigger finger held stiff. Use your left hand to pull the gun forward against your trigger finger.
The recoil bumps the gun backward, your left hand moves the gun forward again against your trigger finger.
It's a cheap and wildly inaccurate simulation of full-auto fire."
So there you have it!
:rapid: 
edit to add: I put this here in the mini forum in case there are those who want to try it out in their mini's (it works!), but I have to advise that if you do do this do it in an area where say Class 3 firearms are allowed or if you know your backstop and range rules- I fire at a USAF range primarily so this is not a concern for me.
From FULL-AUTO.com, credit Rabid Coyote:
"Bump firing is a technique utilized by people who like to make noise and waste a lot of ammo.
Its an inertial firing technique. A rigid finger is placed in the trigger guard without the trigger hand really grasping the weapon. The off hand is used to pull the weapon forward, tripping the trigger against the stationary trigger finger. Constant forward pressure is applied with the weak hand, and recoil causes a pulsing movement of the rifle, tripping the trigger continuosly against the rigid and stationary trigger finger.
Some people can do this at a rate that appears to approach cyclic rate."
Then credited to Snipe17 for the following: (how-to)
"Essentially, you hold the gun about waist level. Let it slide loosely in your right hand with trigger finger held stiff. Use your left hand to pull the gun forward against your trigger finger.
The recoil bumps the gun backward, your left hand moves the gun forward again against your trigger finger.
It's a cheap and wildly inaccurate simulation of full-auto fire."
So there you have it!
edit to add: I put this here in the mini forum in case there are those who want to try it out in their mini's (it works!), but I have to advise that if you do do this do it in an area where say Class 3 firearms are allowed or if you know your backstop and range rules- I fire at a USAF range primarily so this is not a concern for me.