The piston is not really inside the pipette shaft but In the pipette tip. There is no air cushion involving piston and liquid. The piston contacts the liquid straight, and no air is changed for the duration of aspiration. Graduated pipettes are used to measure a variable volume of liquid. https://differenttypesofpipette67395.thenerdsblog.com/34989611/new-step-by-step-map-for-lab-pipette-types