Many types of ball bots have some sort of mini robot rolling
around it's interior. Others have rotary motors connecting
the ball to an inner unit, which has a weight. Not this
Instead, there are several linear actuators which connect an
inner brains+battery component to the outer
ball, not unlike
the arrangement of a "parallel manipulator".
By pushing or pulling with these actuators, the inner unit
can move itself to any location inside the ball.
If the ball is relatively lightweight, and the inner unit
relatively heavy, we can deliberately unbalance the robot so
it rolls in a particular direction.
Rolling isn't very exciting, but consider that the inner unit
can also move itself up and down, causing the ball to
bounce. If the bot is sturdy enough, it might be able to
bounce over obstacles that it cannot roll over.