What is the working principle of hydraulic jack?

Working principle of hydraulic jack:
Composition: the large oil cylinder 9 and the large piston 8 constitute a lifting hydraulic cylinder. The lever handle 1, the small oil cylinder 2, the small piston 3, and the check valves 4 and 7 constitute a manual hydraulic pump.
1.If the handle is lifted to move the small piston upward, the volume of the oil chamber at the lower end of the small piston will increase to form a local vacuum. At this time, the one-way valve 4 is opened, and oil is sucked from the oil tank 12 through the oil suction pipe 5; When the handle is pressed down, the small piston moves down, the pressure in the lower chamber of the small piston rises, the one-way valve 4 is closed, and the one-way valve 7 is opened. The oil in the lower chamber is input into the lower chamber of the lifting cylinder 9 through the pipe 6, forcing the large piston 8 to move upward to jack up the heavy objects.
2.When the handle is lifted again to absorb oil, the one-way valve 7 is automatically closed, so that the oil cannot flow backward, thus ensuring that the weight will not fall down by itself. By constantly pulling the handle back and forth, the oil can be continuously hydraulically injected into the lower chamber of the lifting cylinder to gradually lift the heavy objects.
3.If the stop valve 11 is opened, the oil in the lower chamber of the lifting cylinder flows back to the oil tank through the pipe 10 and the stop valve 11, and the weight moves downward. This is the working principle of the hydraulic jack.