Just my two cents. As a receiver, much like an outfielder in baseball, you are taught to run with your arms pumping and not reaching until the last second. First, if the defender is watching your hands he does not know when to strip the ball if you wait and let the ball get to you. Secondly it helps maintain full speed with no wasted motion. Now, facing the qb, if the ball is above your waste you catch the ball with your thumbs pointed in. If the ball is below your waste the thumbs point out. Simple stuff really.