If you are talking about the thingamabob that is attached on the left side of my rail, that would be a GoGun Gas Pedal, can be picked up mega cheap, i really like it when coupled with my AFG, it allows the thumb to ride on top of it, i put the index finger of my left hand around the front of the AFG, my pinky through the hole in the AFG and my thumb over the 'Gas Pedal'.

It allows me to control the motion of the rifle a little better than trying to grip the quadrail which is quite bulky.
I find in conjunction with the AFG it suits my shooting stance perfectly.

Gas Pedal(R) RS2 – GoGun USA