seeing and hearing
As long as the timer can hear your whistle, the point is fine. If you are in a HS gym that is very loud, it is a darn good idea to raise your hand so that the timer can SEE when you want the clock stopped. (I think people tend to point a bit after blowing the whistle.)
I had a game this year in which we had to decide whether or not the last shot, to tie game, counted and the crowd screamed so loudly that no one heard the horn. Fortunately, I had a clear view of the shooter the ball and the clock at 0.0 before the release. In the absence of hearing a horn that was the best we could do.
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