Hpipes are installed in a dual-exhaust system as close to the exhaust headers (manifolds) as is physically possible.
BackPressure Dual-exhaust systems without cross-over pipes can build up enough back pressure to inhibit flow and reduce horsepower.
Equalizing Pressure Hpipes do not actually carry much exhaust gas. These cross-overs are designed to give pressure pulses inside the system a place to go to keep them from inhibiting flow.
X-Pipes A close relative of the H-pipe is the X-pipe, which gives the exhaust a place to cross over smoothly and helps to increase flow.