Oooops...to answer the original question.
Axle wrap occurs as power is applied thru the driveshaft to the pinion and then rotated into power into the axleshafts. The resistance of the tires to the ground surface causes the pinion to rotate up rather than transfer that power directly to the ground. Soft springs accentuate this problem, and when the vehicle begins to "hop" a bit due to the low tire pressure and limited traction surfaces we run on, the springs will oscillate and the center section nose/pinion/flange/whatever will rotate up and down rapidly. GENERALLY, you will go BOUNCE, BOUNCE, SNAP....been there, done that, twice. Steep sand hills are a great way to bust pinions....
Oh, and the torque applied to the springs is identical on SUA and SOA applications....its the LOCATION of the torque that creates additional spring wrap.
Terminology: A TRAC BAR is used to locate the axle side to side, much like what's on the front of newer Jeeps, early Broncos, and many many other vehicles. A WRAP BAR is generally a triangulated unit designed to prevent the rotation of the axle housing while still allowing free suspension movement. I've got one teched on my website, and there have been many others done. NONE are pertect in their design, however I have no complaints with mine!