In case of electrical transient means for example when you select EXT PWR off. Although APU is running you still get a short break power (blinking). In this case (again not always) as well as in other possible transients (electrical failures for example), if GND CTL was ON it goes off. GND CTL to OFF (ON legend on switch not lit) means AUTO mode is active. In AUTO mode (ON not lit) recorders operate: for first 5 minutes after power applied or at first engine start and onwards on ground / always in flight. Hope this helps.