There are essentially two classes of backbends.   In the first class, the muscles of the spine and extremities are basically pulling the spine upward off the ground into a backbend.   In this class, the arms tend to be down the torso in a similar direction as the feet (but there are exceptions).   In the second class, the muscles of the spine and extremities are pushing the spine upward off the ground into the backbend.   In this class, the arms are usually overhead in the direction away from the feet.   Iyengar yoga teaches both type but tends to emphasize the second class.