Baird's description of how phonic coils operated sounds like a sync pulse to me. There was a black band between frames that produced a 375 hz tone. If it weren't for the presence of the black band (the one constant component) there wouldn't be much point in applying a video signal to the coils. It follows that, since the sync tone was generated by masking the picture, if there was nothing but black being transmitted that would indeed result in little to no 375 hz signal being applied to the coils, likely resulting in loss of hold between scenes.