The rig consists of this:
1 ball is hidden which controls position and squash and stretch. It's pivot point is at the bottom of the ball so that the squash will deform it top to bottom.
The ball you see is a 2nd ball which is linked to the first one. It's pivot point is in its centre so that it can rotate like a regular ball (ie...roll).
An FFD modifier is on the 2nd ball (the one you see) to control the bit of crease you see when it hits the edge.
Other than that, everything is animated using the transform controls only (pos, rot, scale).....there is no fancy rig