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This is called the "up down" on a Bosu ball.. The girls are using their upper bodys, arms etc.. to climb up and down on the bosu ball while at the same time engaging their cores.
The girls are standing on the Bosu ball to work on balance as well as core.. You are keeping your core tight to stay balanced on the ball. They are incorporating a shoulder press as well to again, target several body parts, muscles in one exercise.