Hydraulic car jacks typically come with two cylinders where oil is moved through them by pump plungers. When the plunger is drawn back (pulling up the handle), the suction valve ball in the pump chamber opens and allows oil to move in. When the pump plunger is moved or pushed forward (pushing down on the handle), the oil moves into the cylinder chamber through a check valve. At the same time, the suction valve closes. This builds up hydraulic oil pressure in the body of the jack, and that’s what keeps your vehicle in the air. Because it is hydraulic pressure holding up your vehicle, it is essential that you use a proper mechanical support, like a jack stand, to hold the weight of the vehicle once it is jacked up. Never put a part of your body under any part of a vehicle that is held up by a hydraulic jack alone.