A hook wrench or C spanner is used to correctly tighten or release slotted nuts (DIN 1804).
This is used to adjust shock absorbers and adjust the chain tension on an eccentric rear axle. Certain hook wrenches are equipped with a hinge, which allows you to adjust the pitch size to a certain extent. This is useful for tightening or loosening the headstock of your motorcycle.
Please note that some makes of motorcycle require a special type of hook wrench to adjust the chain tension.