Unlike the later batch of electric aeroplanes that use a 'soft' transfer.
I think the contactors just move quicker. All the electrical switching on the A320 series is relay-logic so it will vary between aircraft.
Guessing that the trick of turning a battery on and off forces a battery to be connected to the DC network for a short period in order that the DC ESS bus is supplied while the electrical transfer takes place.
Always found that jabbing the EXT PWR button instead of a solid push resulted in a messy transfer but possibly placebo effect!