Flatlines work by back spinning the ball - same principle used in the automag Z-frame (I think). Plate pushes down on the top and when you fire the backspin effect gives you better range although at the expense of velocity, I would imagine. You can "dial" in the level of spin you want on the Z-Frame although I read a report that said it's still not consistent enough to help aiming: do you aim flat and let the spin carry the range, or lob the ball if it fires on the usual trajectory?
You also need fairly large bore, decent paint. The flatline and Z-frame are the only ways I know of controlling the spin on a paintball - usually spin is uncontrollable, and when you get a spinner it's anyone's guess where those suckers are going!
Sinner is 100% correct in his points - Tom Kaye at AGD commissioned a load of research into paintball ballistics, because most of what holds for firearms simply doesn't apply to the things we use here. Most of his findings are on the "tech tips" at automags.org if you're interested.