Curious, how does this work?
 

What exactly does aheader do? how does it help performance?

i just got mine and am curious about it,

flows more air. faster you can move in/out the motor faster you go.

eh, makes sense

Not only does it let the air out faster,it also "times" the pulses as they exit the engine.Air doesnt just flow out of the engine smoothly like wind,it comes in pulses.
After each pulse,there is a vacuum behind it.If you make the header pipes the right length and shape,the vacuum pulses from one pipe will help "draw out" the pressure pulses from the other pipe.The more you help the exhaust gasses exit the engine,the more room there will be for fresh air and fuel on the next cycle.....= more power and torque.
It also has the often undesireable effect of increasing the rotaries exhaust noise,A LOT!!!! Be prepared for some serious,dirtbike/weedwhacker noise from your tailipe,unless you run a RB muffler.