You have the freedom to place half balls anywhere which makes each exercise unique. Keep it centered for an even weight distribution or place it to the side and work that side harder.
Place one half ball under the board to create a wobble board and have board tilt in 360o full rotation.
Place two half balls under the board to create a rocker board that will tilt in two directions. Balancing left and right, forward and backward or on a diagonal plane.