The sine wave motion is more apparent when viewed from the side. As a waver steps forward into a punch, the body rises during the first half of the step and drops during the second half of the step. Over a series of steps, the belt knot will appear to travel in a sine wave.
Sine wave theorists posit that, as the body drops, its kinetic energy causes an increase in weight and speed and therefore, power. This is true, but to gain this kinetic energy, energy and time must be expended to raise the body to the increased height. The extra expended energy saps your energy reserves. The extra expended time, slows your attack. Also, the sine wave motion adds extraneous movement to the attack. Any movement that is not toward the target is wasted. Also, as the body drops, it naturally drops toward the center of the earth, not forward into the target. To change this direction and make the drop move in the direction of the target requires the waver to use more energy and creates a slower overall movement.
What goes up, must come down. Since the sine wave movement is up, then down, the end of the movement is at the down position, which must therefore be the normal, stable fighting position. Therefore, to attack, you must first rise to a position that is not as stable as the normal fighting position. This means you must commit to the attack before the actual attack occurs. What if the opponent acts in a way that makes the attack useless? You are now in an unstable position and your options are limited. With the hip snap, during the forward movement, you are always in a stable position, and the snap occurs at the same time as the attack. Thus, if the opponent acts in a way that makes the attack useless, since there is no commitment to the attack, another attack or a block may be initiated.