I've been thinking about this a little more and have a follow-up question:
As NSEU stated, the Transfer Switch will only change the BTBs' position once airborne. So what will happen at liftoff in this example:
After selecting the APU Generator to both busses, the Transfer Switch is moved to OFF. BTB1/BTB2 remain closed. The aircraft now takes off (the IDG's are never connected).
Wouldn't this scenario cause both BTB's to open once airborne? Ok, thankfully the NG will connect the IDG's auomatically if the APU generator fails. Is this what happens?