In air-over-oil cylinders, when the air valve is shifted, the air is allowed to flow down one airline to the air-over-oil tank. The compressed air first fills the empty space in the top of the air/oil pressure tank, and then, as air pressure builds, exerts a force on the oil in that tank. That oil then starts to flow through the lines to the cylinder port, causing the cylinder rod or carriage to extend or retract. When the air valve is shifted in the other direction, the air flows down the other valve line to the other air-over-oil tank, and the cycle repeats itself. Each time the valve shifts, the oil being driven into the cylinder from one tank pushes the cylinder piston towards the other end of the cylinder, and that piston drives the oil on the other side back up the line to the other air-over-oil tank.