I am a little confused on targeting when the runner has the ball. Whether it is a running back or a receive who caught the ball and is now plowing forward. I fully understand if somebody is defenseless, but, if a runner (whether running back, tight end, or scrawny receiver) is in full running motion, how is he defenseless. I mean, the runners are always running and putting their head down as they are about to pummel a defensive back. I would be much obliged if anybody can clarify this rule.