In a fighting stance, the knees are bent and ready to react. You never telegraph a movement by prepositioning the body. For an upward technique, such as a wedging block, as used in To San pattern, the knees drive upward without any initial downward movement. For a downward technique, such as a low outer forearm block, the knees drop into the block without any initial upward movement. For a jump kick, you jump without any downward movement. All movement in any technique should be toward the target. Any other movements telegraph the movement, waste vital energy reserves, and add extra time to the technique.
Hip snapping is done without any initial movement. The hip is snapped from its location at the time the technique fires. Since it opposite hip is automatically cocked when the other hip snaps, it can add even more power to a technique from that side. Sine waving usually requires an initial movement upward.
Using the hips to apply power to a technique is a natural movement. If you tell the average person to push with the right hand, the right side of his or her body will twist into the push. The person will not rise up and then drop into the push.
To test the difference between a hip snap and a sine wave, have a friend step into a sitting stance and extend his or her arm in a fore fist punch. Then you step into a sitting stance and move forward until the friend's fist is touching your nose. Then, without moving the fist, have your friend perform a sine wave bob into the fist. Did the fist hurt your nose? Then, if you enjoy pain, have your friend perform a hip snap into the fist.