These are posts by a manager of Magnaflow..he posts on another board as I do and these are cut/pasted from his posts.
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The X pipe takes advantage of the exhaust pulses that the engine creates as the different cyclinders fire-
As one side of the motor fires, an exhaust pulse goes through the exhaust system, and as it goes through the X, it creates a vacuum in the pipe coming from the other side, which will in turn help to pull the exhaust out of the cumbustion chamber when the exhaust valve opens-
On a stock Mustang, this netted us approx 5-6rwhp on average and around 3rwtq over the traditional H pipe design-
Also, as a result of the exhaust paths crossing each other directly, it also modifies the length of the soundwaves which in turns quiets the exhaust system-
The main problem with the H pipe, is that once a certain RPM is reached, the exhaust velocity increases to such that it is bypassing the balance tube-
The secret with the success of the X though is maintaining the highest exhaust velocity possible, as the faster the air moves, the more efficient the X is (if designed properly) so a straight through muffler will create bigger gains for the X then a chambered muffler will-
On the other hand, as a chambered muffler will reduce exhaust velocity, it will slow the exhaust down enough so that the H will become useful at a higher RPM then it would with a straight through muffler-
Overall, the main goal of an exhaust system is to remove exhaust gases at the highest possible rate, so mainatining the highest velocity possible is key in a good exhaust system-
http://www.magnaflow.com/05news/magazine/05racepgs.asp
Here is a magazine article where they tested an H pipe and an X pipe and the X pipe gained 7.4rwhp and 8rwtq over an H.
We used to make H pipes for Mustangs, that was selling well, but we felt the X provided enough benifits to make the switch.
magnaflow exhaust 
