a DROP FORWARD will move whatever system you've got forward and down. so if you're runnin co2 it'll move your current on-gun setup forward. with this you'll need a seperate donkey (bit the co2/ air preset screws into)
A DROP LINE will do the same but also incorporates the donkey into which your co2 or preset air will screw- basically does the same job as above but much much neater and in one piece.