Why should your body be forward in acceleration?
Being forward allows the knee to drive forward, stay low to the ground, and stay bent on ground contact. The bent knee provides a shorter lever that allows the glutes to move your currently slow-moving, heavy body forward from a stationary position. The shorter lever of the bent knee is like using a small-radius front gear on a bicycle to accelerate or pedal up a hill. Another analogy is the difference between the arm movement in a military press compared to that of a lateral raise - more weight can be lifted in the military press because the arm is shortened with a bent elbow. While on this topic, remember that the glutes are the primary muscle to be used. I’ve heard coaches say to emphasize the quadriceps, but the quadriceps are not going to move the body forward as efficiently as the glutes.
Another reason for being forward is to have the torso in a strong alignment in the direction of acceleration, and with a strong connection to the hips. If you try to accelerate with an upright torso, your hips will move forward and you torso will be delayed or lean back. This is a soft connection of the torso and hips, and it prevents the force created by the glutes from being completely transferred to the torso. To strengthen this connection, keep the abdominal muscles crunched, the back slightly curved forward, and look at the ground. Be careful not to bend forward at the hips with a straight back. This creates a forward position without the crunched abs and may result in falling forward. The crunched abs also assist in knee drive and scissoring the legs in the air.