It is triggered by a G-switch when the rocket lifts off the pad, and is somewhat like an altimiter in htat it can be set to deploy the parachute 15 seconds after activation (just an example) or airstart motors.
Actually, the Trans-Beep does *not* beep on the pad. It has an optical sensor on it so it stays silent until it is deployed with the parachute. I have one and I love it, but it's heavy enough that you only want to use it on very large MPR birds to HPR.