Unthinking reaction to tap in surgery? (4-4)

I believe the answer is:

knee-jerk

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But I can't explain it at all! Could you help me understand?

(Other definitions for knee-jerk that I've seen before include "Involuntary or hasty (reaction)" , "(Of a reaction) automatic and unthinking" , "Thoughtlessly spontaneous" , "(Of a response) automatic and unthinking" , "Sudden and ill-considered" .)

I've seen this clue in The Times.

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