Why do oversized PV covers do what they do?

[motogroove]

People are changing their power delivery using the oversized covers. Better low end I believe is the goal.

The action of the rotary valves that (I think) port the exhaust over into the PV cover(s) is kind of hidden, and I'm trying to understand the how and why.

When is the PV chamber in use? High revs or low?

My crude understanding is that the PV chamber volume is used sort of like stretching the exhaust like a trombone. But I could be way off and I still don't quite get the timing, etc.

Anybody want to take a crack at explaining it?

Thanks a bunch!