The PWS seems hollow from the pics taken by PT630, but even if it is hollow, it should add some weight to the frame, as the cross section is broader.
Anyway, I think it is mainly a gimmick in that to add weight they should not add any visible parts, they could just do it by using different material density in a straight or box beam.
But I must say I do feel the difference between the classic Wilson racquets with PWS and other brands without it.
Would PWS have any other effects than adding weight. I mean, the strings are slightly shorter around the PWS sections, would that have any effect?