IIRC, it starts transmitting as soon as the ATSU detects you have departed and stops when it detects you have blocked on. We leave ours in Auto all the time.
This is confirmed by the fact that when we had 3 knots as the parameter for the recording of the Out time, we used to get ATC asking us to switch the transponder on if it was a slow push. Now we use the park brake off as the parameter, we don't get the ATC requests.