Punching versus Hammering
When you use a hammer to drive a nail, the head of the hammer moves in a large arc; however, the other end of the handle only moves in a small arc. When sparring, a punch should use the same motion. In your guard, the lead hand (head of the hammer) may move randomly to keep it in motion to confuse the opponent, but the elbow (end of the hammer) should stay relatively stationary in front of the body, protecting the lower ribs. When ready to punch, you "drop the hammer" by suddenly moving the fist toward the target. In this motion, the fist makes a large movement while the elbow moves relatively little.