The cocknob is used to control the flow of oil into the gearbox. When the cocknob is turned, it opens a valve that allows oil to flow into the gearbox. When the cocknob is turned off, it closes the valve and stops the flow of oil.
Cockknobs are typically used on gearboxes that are difficult to reach, such as those that are located on top of a machine or in a confined space. They are also used on gearboxes that require frequent oil changes.