Originally Posted by
FlightDetent
If the EXT PWR remains connected, there is probably no technical reason to wait.
I found that if EXT PWR is available the time lapsed between me shutting the APU OFF and performing the securing actions is well above 2 minutes anyhow. This particular piece of SOP is written for the case when you leave the aircraft withou GPU connected, I think.
FD.
The two minutes wait is for the APU flap to close if done earlier thare will be smoke in cabin during next start. Whenever it is closed batteries can be switched off.