They work to boost bottom end response by increasing the resonating volume of the exhaust, I think?. They are only in effect when the PV is closed, when the PV opens at higher revs that area is blocked so no longer effects. It gives the bonus of adding length to the pipe for bottom end advantages with out the disadvantages of a longer pipe, which is obviously harder to make fit on the bike.
Someone else will have to chime in on the mechanics of it, I havent covered that part of my degree yet
