The piston (number 5 in Figures 2 and 6) is the larger diameter cylindrical part located within the hydraulic cylinder (4 in Figures 2 and 5). The piston effectively splits the hydraulic cylinder into a left side and a right side. Oil cannot pass from one side to the other past the piston. It must exit and enter the hydraulic cylinder through the hoses attached to the directional control valve. The hydraulic oil pressure is exerted onto either side of the piston in an alternating fashion so that a back-and-forth movement of the piston and plunger assembly is generated.