A hydraulic jack functions by utilizing hydraulic fluid that is pumped through the mechanism by force exerted by the user. When the handle is pumped, the fluid that begins located in the reservoir of the jack is pushed through the valves and into the bottom of the load-bearing cylinder pushing up the load-bearing cylinder as well as whatever item is on top of it (car, equipment, etc.) If you are looking for a great visualization of how a jack functions check out this video