The HPx line connects the two heads to help balance the high pressure oil system in each head. It does not create more horsepower, but the majority of owners see a slight to significant decrease in engine idle noise. This is due to the Helmholtz resonance that is created from the injectors firing so quickly, and the hpop cannot react quick enough to balance/absorb on its own from back pressure spikes within the system. So when the tiny spikes happen, the hpx line sends the pressure back and forth through the heads instead of hammering on the pump itself.
