Sounds like a resonator to me, it may ad a Lil volume to the pipe and from what I understand it does work kinda like an expansion chamber with the scavenging effect.
Well I found there was a bit of difference in bottom end between a stock head pipe, and the power bomb. Especially from idle to about 3500-4000rpm. But it also kinda seemed to make the top end fall flat...
Do I think that a power bomb is worth the money.. No I don't. I think their worth $80 bucks max!
But if your rolling tight trails with steep slow hills, then ya it's worth have'n.