The belt pack has a microphone attached to it. The officials string the wire up through their shirt and attach it to a small clasp a few inches down on their lanyard.
The system listens for three distinct tones of the Fox40 whistle. No other whistle will stop it and breathing won't either (the guy was either joking or had no clue about how it worked).
And, Mark, it cannot tell if you have an inadvertant whistle or not, but a red or green light comes on the belt pack depending on whether or not the call you made was the right one. Also, if you stare at the cheerleaders too long it gives you a small electronic shock.
