[just another ref]

Originally Posted by rsl

From the manual:

D. Resuming Play:
1. At the warning signal (first horn) for all time-outs, and the intermissions
between the first and second and third and fourth quarters, the
officials will step toward the nearest team huddle and notify the coaches/
bench by raising an index finger and saying "first horn."

I do it, but I find this to be, for the most part, useless.