It creates a vacuum in your exhaust system that sucks the exhaust ouf of the engine, so the engine doesn't have to push the exhaust out on it's own power.
I think you are gaining torque on an x-pipe or h-pipe.
Both will get you better torque and horsepower, though as someone mentioned, the H-pipe is known for creating better torque numbers. And either one will make a huge difference when added to an unbalanced dual system. While I've had h-pipes on several cars over the years, my only experience with an x-pipe so far has been the one installed on my Mach. One thing I've really noticed with the x-pipe is the exhaust has a less "muscular" tone... not sissy mind you, just not old school musclecar. Another thing is that the exhaust is quieter than I expected and quieter than similarly built cars I've seen/heard, and it's definitely quieter than an h-piped car.
Most importantly which X pipe you are using and some are definitely made better then others-
I guess the best way I could put it is when I first finished my Shelby, it had an H pipe and Flowmasters on it- (yeah yeah yeah I know- they were on the car when I bought it)
After installing the Magnaflow X pipe and mufflers though- my RPM jumped up 150-200 RPM at idle because the car was breathing so much easier-
I forgot the exact numbers, but I picked up torque and hp across the entire power band-
X pipe is definitely quieter though then the H pipe- the sound is more NASCAR and less muscle car though- best way I could describe it-
On my 66, it is so quiet at idle, and so much louder above 4000RPM, I can usually scare people pretty good...lol
Working where I do, I could run an X or an H as we make both- after having both I will never run an H again though-