They were air driven and may have come on at a certain speed, as in a dive. Apparently they could be switched off but I've only found one source saying it was possible.They dropped the speed of the Ju 87 by 10-15km/h. and when speed means life, the later variants discontinued it. From the one source, it says that bombs were introduced that had a siren attached to the fins that made a wail but I've never seen a photo