The ball against the wall with the rope is a great core strengthening exercise, working on resisting rotational forces, the same as you would when checking out of a jump. With the skater in the video, I would use a smaller ball, as she is using too much trunk and shoulder rotation in the exercise. This means the weight of the ball is too hard for her. If you try this exercise, you need to keep your abdominals in to maintain a neutral spine position. Also, in the exercise where this same skater in lying down and coming into a sit-up position, throwing a medicine ball at the wall, she has way too much lordosis in her back (excessive arch). Again, the exercise is too difficult for her, as she can't stabilize her spine with her abdominals. The spine should not curve into a lordosis. Very important for anyone trying this exercise!
Matt has some great ideas out there; it's too bad that off-icestrength.com folded.