There are a range of training methods that teach a student to counter a knife in self defence.
Students learn how to avoid being stabbed or cut, escape from a hold and apply a chin na against a knife-wielding attacker.
Many of the basic drills can be used to great effect against a knife.
Self defence against a knife offers no guarantees in real life.
In truth, success is a more a matter of opportunity, awareness, presence and technical skill than anything else.