bouncing volt and ammeter

Maybe it would help to look at battery and alternator operations.

Batteries suck power in proportion to need if it can be supplied. Time lapse photo sequence of low battery on charger.
Alternator's maximum potential power output goes up with rpm. Output chart for an alternator

So it seems like you are seeing system voltage going from being supplied from the alternator to the being supplied by the battery. Then as rpms go up, the alternator produces enough power to run the ignition and recharge the battery and voltage rises from the battery's to control by the regulator.