armchair official here

In addition to the following I also noted a rule that said that raising an arm to block the sun, but not waving it, was legal.
It seems to me that if you wanted to coach a receiver on a signal to tell teammates not to touch the ball the 2 hands low pointing at the ball, and not waving them, would be a great way to do it. This signal would not be legal if the players hand(s) are waving

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Valid Signal
ARTICLE 2. A valid signal is a signal given by a player of Team B who has obviously signaled his intention by extending one hand only clearly above his head and waving that hand from side to side of his body more than once.
Invalid Signal
ARTICLE 3. An invalid signal is any waving signal by a player of Team B: