basically it connects the two pipes so that the back pressure is equal. it will help you.

 

i just put an H pipe on my duel set up, it mellowed out the tone alot, i couldn't feel a diference but i can't feel anything less than 30hp it seems.

an x pipe is supose to be alot better than a H pipe but a H pipe is better than nothing, if you allready have duels a H pipe would probably be the easiest.

 

What does it do? I have true duals. Would it help me any?

99 ext. cab silverado 5.3
3.42 (stock)
K&N FIPK
Jet PCM
True Dual 40 Series Flowmasters
160* Powerstat
Taylor Spark Plug Wires
GMS Mass Airflow Sensor

 

X or H both are balance tubes which are meant to help the scavanging effect of your exhaust system. X-pipes are less invasive towards flow and tend to yield better Peak HP...H-pipes actually cause a bit more turbulence but to the effect of netting slightly more backpressure increasing TQ.
Honestly I think all too oftern both are arbitrarily placed where they fit best over wher they work best by most exhaust shops. On my 95' ImpalaSS I had the H pipe welded and removed 4 times until I had the best location. The best location for it proved to be a place not very accomodating to ground clearnace but still netted almost 6 RWHP more over the original location which best "fit" under the car.

I know professional race vehicles spend hours of dyno tuning and complicated math to find the "best" location for these balance tubes.

Bottom line...I don't think I have even seen an improperly placed balance tube have a negative effect on the vehicle.