Ju-Jitsu has many defences against knife attacks, involving locks, throws, and strikes. However, as was once recommended in an Aikido lesson, the best defence is to hide or run away. A life is a very dangerous weapon in the hands of those with ill intent and much of Ju-Jitsu is about awareness and moving away from problems.
Knives can be used to attack many parts of the body with stabs or slashes and someone who is used to knife fighting can mix in grabs and other attacks to try to prevent you from defending yourself.
However if there’s no way out of the fight, you’ll need to know how to avoid getting hurt if possible. The practitioner will start this practise against rubber or wooden ‘knives’ and move to blunt metal knives as skills improve and to create realism.