The pinion gear when pushing against the ring gear which turns the axles then the wheels meets with resistance.
So as the pinion gear turns against the ring gear (and rear diff assembly, axles, wheels) the resistance of those parts to move causes the pinion gear to want to ride up the ring gear in the opposite direction that it is trying to turn the ring gear.
Being the pinion gear is fixed to the rear diff/axle housing assembly it is not free to rotate in the opposite direction/climb the ring gear. So that reverse force will cause the whole rear diff/axle assembly to rotate and twist.
The Torque Tube's job is to provide rigid support to prevent the rear end from twisting when engine torque is applied to the rear end.
Rear leaf springs aided in controlling the twisting of rear end.
Rear coil springs without the torque tube requires the use of the 4 control arms to control the twisting motion.
There were some modern/late model autos that had an open drive shaft with a steel beam that bolted from the trans to the rear housing to act as a torque tube. Jeffrey
