Mine is incorporated into my vegistroke unit, so fuel lines run from the veggie pump to the back of the heads and there is a check valve going both ways. So when it's on diesel, diesel is pumped into the front of the heads, runs out the back of the heads and hits a check valve in the vegistroke manifold (holds the pump, filter, temp switch, pressure switch and the fuel pressure sender). So I'm reading fuel pressure at the absolute farthest point away from the pump behind all 8 injectors. Basically it acts like a x-over line for the rear of the heads. When it's on veggie, it pumps up into the back of the heads and hits check valves at the front of the heads that wont allow veggie to go into the diesel side.