I'll keep it basic but informative. The x pipe is designed to merge exhuast pulses into one area. The result is increased heat at one point in the exhuast flow which leads to increased flow and thus more power. However, because of this increased flow, more backpressure is lost. It's also a fact that backpressure is important for torque numbers. This is why many experience a loss of torque in the lower rpms with the addition of an x pipe. However, their is usually a better power gain on top end compared to the h-pipe. The h-pipe uses the simple straight flow technology and dumps the exhuast pulses into the mufflers. Since it lacks a "merge center", the exhuast pulses will flow, but not as fast as an x pipe will. Since the exhaust pulses dont flow as fast, there is more backpressure with the h-pipe and thus better low end torque numbers. However, in higher rpms, the h-pipe will have a bit more restriction compared to the x-pipe which will result in the x-pipe in most cases than not producing more power in higher rpm levels.