He's talking about a crossover pipe, so that off the headers of a V8, you have 2 short straights, a h pipe connecting the 2 sides, then the duals back from that. It serves the same function as a crossover pipe in scooter exhaust, creating an artificially larger volume system than would be possible with each header feeding an individual pipe to termination. Its a torque improver due to the better scavenging at low velocity, and a horsepower improver due to the larger effective volume of the system, somewhat like getting a 2 1/2" system to flow like a 3" would. The same thing is accomplished more or less by the cat chamber-it is a common dump, though it's back a ways, it's pretty damn close to the headpipes, and accomplishes the same thing. There are also connectors at the headpipe, so that 2 pipes are shared by each jug. This is an assumption on my part-they are there for this purpose, for strengthening the head pipe, or both.
Prof. rad