That's pretty much what it is. I believe the inlet/outlets into the chamber are slightly flared into somewhat of a velocity stack trumpet shape to help speed the air along.
According to Mac the concept works by creating a hot chamber which helps with scavenging by drawing the exhaust gasses out of the tubes coming out of the headers and speeding them along into the colder outlets out of the car.
Probably not much if any with stock cats. I have not head how a prochamber spliced into the stock cats sounds, but I have read from more than one other poster that is sounds very similar to the stock H-pipe. Each one said it was slightly deeper, but also a little quieter.
I think prochambers sound awesome and make good power. To try and say wich is better and makes more power the h, x, or prochamber is apples to oranges. It all depends on your setup and sound preference.