Knuckle jars are typically used where there may be a length restriction.
The knuckle jar combines the functionality of a knuckle joint with the versatility of a mechanical jar.

Applications:

The knuckle jar can be utilized when running into deviated wellbores.
The knuckle jar can also be used in conditions where toolstring length is restricted, such as rigging
up remotely with restricted lubricator.
The knuckle jar can be used in place of typical mechanical jars if there is no concerns of heavy jarring loads.