The ends not welded to the mufflers are capped, what it does is bounce the sound back of a certain frequency (determined by what frequency you are droning at) and cancels out that frequency. And since the other ends are capped the exhaust doesn't flow into it as air takes the path of least resistance. They did not change the sound at all other than eliminating the drone. To me, no drone is well worth the few extra lbs.
These work purely based on their length and one end being capped. The objective is to "bounce" sound back into the exhaust system at 1/2 length of the frequency your droning at. the forumla used is:
(1100/y)/4=y then z=(y)*4
y is the frequency at which you are droning (i used an android app to find this)
z is the overall length the "j pipes" need to be
(mine are 30" to target drone at 110hz)
they don't HAVE to be in the shape of a J, they can be any shape, and tie in just about anywhere on the exhaust from my understanding. This also works on any engine, v6, v8, i4, boxer, etc.
this thread goes a little more in depth on the science behind it