How does the aneroid wafer expands? (ASI)
The aneroid wafer is just a metal balloon.  Add air into its interior, above ambient pressure, and it expands.

The outside of the wafer is exposed to ambient static pressure.  The inside of the wafer is plumbed to the pitot inlet.  Ram air into the pitot raises the pressure within the pitot plumbing to above ambient, which being connected to the interior of the wafer, inflates it.  The movement of the inflating wafer is geared to the airspeed indicator and voila, ya got yer speed indication.