Here is a video of an elite ski racer performing "Roll Back to Squat Hops".
She is training for coordination (as well as endurance) as she coordinates her arm swings with her jumps.
In the next video she adds speed to the exercise.
She still focuses on maintaining her coordination as she increases her speed.
Please Note :
-the athlete's fight to always coordinate the swing of the arms with the jumps
-the athletes fight to nail a perfect 90 degree turn- by jumping high and therefore have time to nail the 90 degree turn