Hello Flyman35,
The aim here is that voice recorder is operating when you read the checklist. During preflight GND CTL is selected to ON. When you select EXT PWR to off, GND CTL might go off also (not always) due to power brake transfer. Then you have to press it again in order for the checklist to be recorded. If you don't do it, recorder starts at the first engine start, so the checklist reading might not be recorded.
Regards.