In effect, by doing speed training you can improve movement speed, conserve energy, and increase overall longevity of performance. Posture - Good posture allows for greater transfer of force to the ground that helps to move the body in any direction. Newton's Third Law of motion says that for every action there is an equal and opposite reaction. This holds true here. As we apply force to the ground, the ground in effect pushes back.
Final result is our body is projected according to the direction of force applied.
Think of your body as a pogo stick. With the stick straight it bounces along with no problems. If we bend the pogo stick in the middle it's very difficult for the stick to work to it's maximum potential. Step down to the ground - We should apply force away from the direction that we want to move. To move forward we push backward, backward we push forward, to go left we push right, and right we push left.