Can I bring the ball behind my back?

There is no restriction on bringing the ball behind your back. The images below show the figure 8 style that has always been legal. It allows you to get your core into the pitch. This style emulates the throwing of a discus from a stationary point. You coil , then uncoil releasing the ball as low to the ground as possible. This allows a longer arm swing = more speed. The drive forward with the lower body. But with all the moving parts it is harder to control. However, you must square back up on release as shown below.

OLYMPUS DIGITAL CAMERA Discus

Traditional style with ball rolling off fingers with downward rotation.