The movies make knife wielders look like an easy disarm. You are 100% correct - a man who knows what he’s doing with a knife can kill even a trained responder by landing a fatal swipe in seconds.
Yes. In a normal fight a trained fighter might concentrate on the torso, watching for the movements of shoulders and hips which can telegraph an incoming blow, but generally keeping your fighting attention centered on the opponent's center of mass.
Say attention 60% on arms, 40% on legs - or something like that. You also watch your range - footwork, etc.
In a knife fight you need to change that focus balance to more like 75 to 90% focused on the knife and the arm that wields it. Most likely the attack will come from there. You have to either block it, deflect it, or otherwise get out of the way.
It is by no means easy. Better if you are very very quick.
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