Referee's Signal Number 15 - Sideline Warning
This signal is used by the Referee to signal a Sideline Warning, and is used to warn the sideline personnel that they are spilling out of the team area and getting in the official's way.
There is no other use for this signal.
A sideline will receive two such warnings, after which the next infringement results in a 5-yard penalty and each subsequent in a 15-yard penalty.
Hold both arms completely extended to each side and parallel to the ground, and with an unhurried motion bring them forward by about a foot. This should be repeated.