""But what happens when switching between the other modes?""

There must be a way to hook up the switch to a light (or meter) and a power
source and see if it does it.  Battery power to inputs, output to a grounded
light.  Slowly turn the switch and see if it flickers.

I'll bet that if the 'both' position is located between the Batt1 and Batt2,
it will be make before break.  An 'overlapping' of the contacts.