Shamu,
A "drop forward" is a fixed bar attaching to the bottom of the grip frame that moves the air system formard.
These have a slide fitting on the bottom that a variable output airsystem (angel a.i.r., flatline, etc) slides onto.
If you use a screw in type air system (pure energy, bulldog)
or CO2, then you need to slide a "Cradle" on to the drop forward that the airsystem screws into.
This is much like a standard bottom line fitting in function, but typically the cradles that many companies produce to accompany their drop forwards are nicer and bring the bottle out paralell to the gun, rather than at an angle.
hope this helps!!
Ian