This clip is purely demonstration, you wouldn't punch or kick thin air with full force, and you don't want to hurt people you are trying to teach either. Power comes from the feet, and the body alignment the waist barely needs to rotate when using the JKD bijong stance. If you start in a natural stance it would then you would use your waist more to throw the strike and then move into stance. To someone who has not studied JKD it looks weak, but that is the beauty of the art, it's simplicity. Other arts like Muay Thai require large swinging kicks to cut down the target. Bruce Lee when creating JKD studied human biomechanics to create JKD, thats why it looks so simplistic.