A stripping knife is mainly used for removing thick cable jackets. You can see how to do this here:
Setting the blade depth: First you determine the thickness of the cable sheath. Then set the blade to the appropriate depth, usually with the help of an adjusting screw.
Putting on the tool: Most stripping knives have a clamp or pliers for this purpose. This allows you to fix the cable at the desired depth.
Cutting the sheathing: Finally, turn the stripping knife around itself once. The blade cuts through the entire sheath once so that you can easily remove it.