you want to restrict the movement of the arm as you don't want to give them too much movement or flexibility or they may squirm their way out. Elbow and shoulder locks work well as they immobilise the arm. Personally I would strike at nerve points on the hand or inside of wrist to force the hand open and therefore drop the knife. You can also try finger/thumb locks to disarm the knife too. I like to teach Thumb locks because "thumbs are your chums"
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