handled an AyA at the cabelas here and it had a small lever on the left hand side of the sideplate? what is it and what does it do?

It's a take-down screw to remove the locks; the "lever" replaces the head of the slotted screw that runs from the left lock to the right lock.

One of those cool traditional shotgun things that can buggered-up by someone who doesn't understand what it is and starts to mess around with it.

It's called a hand detachable lock or lock plate.

AyA copies this from high end British designs.

It is a nice feature and beats screws with traditional slots as it allows you to take the side locks out for cleaning and maintenance without the need for a screwdriver and lessens the chance you will bugger the slots with a cheap or ill fitting tool.