Axle wrap is the tendancy of the axle to twist as you accelerate. Imagine you are holding a wheel that is trying to turn to push the truck forward. If the wheel doesn't move, then the axle will turn in the opposite direction. The same thing happens all the time to some degree. If the springs have a lot of flex front to back, the effect can be pretty noticable and even start breaking things like u-joints.
The further out from the springs the axle is, the more force it can apply in trying to twist. If you continued the curve of the leaf spring into a complete circle, the farther away from the middle of that circle you get, the more leverage you get to torque on the spring. Spring under axle puts the axle closer to the center of the circle and cuts down the leverage. Spring over axle puts the axle further out so something else has to be done to compensate. I suppose this is probably why a lot of spring over axle setups look more flat, they make the spring arc longer/flatter. Instead of moving the axle further into the circle, they make the circle bigger.
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