To apply mass to a technique without leaning or reaching, which causes instability, we must snap body mass into the technique using the hips. To illustrate this motion, stand in front of a target pad or bag and hit it with hook punches. Now, from your guard position, raise your elbows upward to the sides so the fists are in front of the face with the forearms parallel to the floor. With the arms locked and upper body locked in this position, hit the target with hook punches without using the arms or shoulders; only use a rotating snapping motion of the hips. With a little practice, you will find you can strike just as quickly and powerfully this way as you did using your arms. Now, if you combine the hip snap with the motion of the arm and shoulder muscles, you can strike with maximum power.
To illustrate the hip snap (or in this case the hip roll) with a kick, extend the leg in a side kick position and hold the heel against the target. This is the point of impact for beginning students. With the leg still extended, roll the kicking hip over and downward, snapping the body mass into the kick. With what seemed to be a fully extended kick, you will achieve even more extension and more striking force.
Most people do not use their hips. They have to be taught to lift using the legs (and hips) rather than using their backs. When they take a step, the hip and leg move at the same time, which means their mass moves with and at the same speed as the foot, which means they fall if the foot slips, and they have no power in the step. When stepping, if the hip trails (lags) the foot, the mass is not committed, so, if the foot slips, you may still maintain your stability. This way of moving is used in Judo. If you commit your mass into a step and the opponent sweeps the foot, you fall. If the hip and mass lag the foot, if the foot is swept, nothing happens. Since people have not used their hips properly for years, it takes awhile to unlearn the improper way and learn the proper way to move their hips.