It's a pipe running between the duals.
Usually close to where they make the turn to horizontal from the header exits.
Your exhaust would look like an H where the crossover would be.
If I can find an old article that explains where it should be placed, I'll let ya know.
Really will help get some of the bottom end torque back.
All street run vehicles with true duals should have one.
This is the best I can find right now.
"H- and X-tubes for dual exhaust--
H-tubes balance the backpressure between the two sides of your engine, and even out the complex pressure waves that are flying around in the pipes. The result is enhanced low-end torque, better overall performance, and better sound.
X-pipes do a better job of this than H-pipes, because there is no distance separating the pipes with an X. But some cars, like my 'Bird, have no way to fit an X underneath.
H-tubes should be at least 3/4 the size of your exhaust pipes, and located in the "hot" area of the pipes behind the header outlets. To find that hot area, one suggestion is to spray a line of ordinary white paint down the pipes after the headers, and go for a good run. Install the H-pipe somewhere in the area where the paint burns off the pipes, equal distance from each header collector. Use the same technique to determine where to place an X-pipe."
