Shoved in with my VDE toolkit is a small kitchen knife, I never go to a job without it! It has small serrations along the blade. The beauty of the serrations is that you can apply very little downwards force, just run the blade back and forth, as you get close to hitting the conductors, slow right down. It won't just unless you move it. It is
the best knife I have used for getting through the outer sheath* on a cable without knicking the insulation on the cores.
<pedantic electrician>
The outer of a cable is called the sheath, not insulation.
</pedantic electrician>