The pumping operation starts with the discharge of the concrete, usually from a ready mix truck, into a hopper large enough to hold a small supply of fresh concrete. In the hopper an agitator keeps the fresh concrete flowing smoothly into the pumping cylinders. Concrete piston pumps operate on the same principle as a twin cylinder reciprocating engine, in which one cylinder draws concrete from the hopper on the return stroke and another pushes it on the forward stroke into the line. Pistons in both cylinders operate in opposite directions so there is constant pressure on the concrete in the line and uninterrupted flow. The pistons are driven by hydraulic cylinders powered by a hydraulic pump. A synchronized valve enables concrete from the two cylinders to go into one pump discharge line. This valve is often used to distinguish one type of pump from another. A few common valve types are gate, rock, swing tube, ball, or C tube.