go check out the Camera Project's video from when Panasonic released the V2.0 firmware. they explain the new Auto shutter mode added and how it woks:
It doesn't engage reliably with all lenses tho, we've had reports here that it basically doesn't opt for e-shutter at all with the 100-400... Helen has commented how, at least based on the display, it also behaves erratically with the small 35-100.
Well, 'erratic' might be the wrong word if, say, Panasonic have tuned Auto Shutter to behave differently with each Panasonic lens, in an appropriate manner--- possibly taking other settings into account too. If that were so, and I am only suggesting it, then 'erratic' could be replaced with 'smart'.