Actually it is a pan hard bar, (track bar) the keeps the rear end centered between the chassis rails in the back. Due to the long springs, 42", and the large spring shackle bushings, the MH has a lot of force that moves side to side, but the tires are fairly fixed to the pavement, so you get the tail wagging effect. The pan hard bar prevents the side to side movement of the MH over the rear end, eliminating the tail wag. As mentioned, this makes a great improvement, and should be the first improvement made to improve handling.
Wildmanbaker