EMS back-up battery switch

Under normal operation, the A Gen powers the ECU and the B Gen Is connected to the Airframe Battery.
Closing the Backup Battery Switch connects the Battery and the B Gen in parallel with the A Gen and the ECU.
This places ALL power sources in parallel.
Nothing Bad happens, but you no longer have any isolation between the two Generators.

No Harm, No Foul.

The Amber Lights is just an Idiot light to remind you that the Switch is ON.
I have an Amber Light on my Start Power Switch to remind me to turn it OFF after starting.

I agree with Bill. Nothing bad will happen and there is no reason to do this unless needed.

Thanks for your previous reply's

So does ( No harm No foul ) mean that the battery is still being charger ??

Yes, the battery is still being charged. (By BOTH Generators!)
And the ECU is being powered by Both Generators AND the Battery.