They are supposed to smooth out the throttle band if I am not mistaken. I don't think I have seen anyone who thinks they are an important hop up. Most think they are a waste. I know MIP makes one.
The way it seems, it squeezes some extra air into the carb intake to do what Dave just said- smooth out the acceleration and overall powerband. I have one on a .21 and I doubt it helps much.
I'm taking this off topic, but has anyone played with the perry pumps? (Nitro fuel pumps that use enegine pressure to pump a constant pressure into the engine) They any good? Something to the tune of $100 for the complete system.
I think maybe because that engine is easy to drill and tape the crankcase (it's square). I've seen other people do it with a hole drilled and taped in the backplate.
The perry pump should nix the fat to lean swells caused by the level of fuel in the fuel tank. It should also smooth out the low fuel pressure issues at low RPMs. You won't, however, see me drilling a hole in my .21 to find out.