In this photo, you can see that the athlete initiates the jump by taking off with one foot. This is important because you need the momentum you have gained from your run-up to jump a longer distance. The speed you gain from your run-up creates the force and power necessary to push off with one foot and propel yourself forward into the sand pit.
On the other hand, taking off with the two feet, stops the momentum you have gained from your run-up and slows you down. This in turn affects the distance you jump because you are using less force and power to propel yourself forward into the sand pit.
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