what exactly is it that the torque arm does?

 

try here

 

how does it work?

 

Imagine what would happen if it wasn't there.

If you look at the car from the left side, when the engine tries to accelerate the car, the tires get rotated clockwise. The rear end housing attempts to move counter-clockwise with exactly the same force as the wheels exert trying to rotate clockwise.

Somehow, the rear has to be held in place. In a 4-link car like a G-body for instance, there are both upper and lower control arms; the uppers go to the top of the housing, the lowers to the bottom; and the rear cannot rotate. In a leaf spring car such as a 1st or 2nd gen F body, the spring itself does this; and a favorite problem one has to deal with as a result, is "spring wrap", where the rear tries to wrap the spring around itself. That's what those "slapper bars" you always used to see on those and Mopars (same design) are supposed to avoid, by providing something to limit the rotation of the housing by hitting the front spring mount or something.

In these cars, the torque arm positively locates the rear end from rotating. The suspension will not work without it. When the car tries to accelerate, the torque arm literally tries to lift up on the center of the car.