X pipe H pipe or side by side
What may be a bit confusing is thinking about back pressure in the equation. The H or X is far enough up stream it is still more a factor of velocity and pressure differentials.
"back pressure" isn't a thing. It's a phrase, but it's not based on reality.
The notion of back pressure comes from the fact that lopping off exhaust or going to too large of an exhaust can lose low-end torque on some setups. The reason for this has very little to do with 'back pressure' and everything to do with momentum.
Short exhaust systems hold too little volume, larger volume = more mass = more momentum. The momentum of the gas stream is what causes the better scavenging.
Too large of an exhaust has to do with gases cooling off as they expand. A cooler charge has less volume, and will also move more slowly. As a result, momentum is also decreased by too large of an exhaust system.
This ignores pressure waves, by the way - that's a whole other ball of wax.
Having an H or X allows the momentum in the exhaust column to be applied to both sides of the engine. Effectively doubling the scavenging effect on any single cylinder in it's cycle.
It's most pronounced down low because the effect becomes overshadowed by other interactions in the intake manifold at higher revs.
