Torque arm function

Can anyone point me to a link or post a diagram of exactly what function a torque arm plays in our cars?

Is it there to keep the axle in line with the drive train? (rotational)

It seems like a simple concept, but for some reason, I don't get it...

It keeps the pinion shaft from rotating upward on acceleration, in response to the torque at the wheels. Check this article:

http://www.popularhotrodding.com/tech/0604_rear_suspension_guide/pictures_diagrams.html

http://images.popularhotrodding.com/tech/0604phr_susp_22_z.jpg

Thank you Fred, that article explained it perfectly for me.