I think it might be a combination of both, but lean towards torque. We did a lot of testing with John Stranahan like 10 years ago on dogbone angles. From our testing, we noticed most of the "straightening tendency of the drive shaft" happens when you apply power.
The effect can be achieved by having dog bone sweep and by moving the diff up or down. Typically when the dogbones are swept forward in the front the car will push the arms down and accelerate better on power which assists in rotation (more on power steering) which is better for slower classes. When they are swept back, it makes the car mellower and easier to drive, which assists in the mod classes.