Sure, sure, to test your reflexes. Everybody knows that. Isn’t the delayed kick in the pants when the doctor turns his back a slapstick mainstay? Harpo invented it. Or was it Curly?

Anyway, testing reflexes is the objective, but why hit that little sweet-spot below the kneecap specifically? Why doesn’t Dr. Feelgood just lob a quick sucker-punch at you to check your flinch factor?

For starters, the knee gets top billing in the reflex test for one simple reason: it’s easy to access. You’re sitting on the table in that backless gown, fighting humiliation and cold. It’s merciful that nobody’s asking you to hop on one leg while you pat your head and rub your belly. There’s your knee in all its unadorned glory. Sit tight, this won’t hurt a bit.

Now, that quick tap should quickly fire your neuromuscular system, causing a rapid stretch of those adjacent muscles and triggering nerve receptors in the tendons. This kicks off a eyeblink-fast nerve impulse transmission up your spinal cord, where it should trigger a reaction to contract the muscle that was just stretched.

Ideally, a muscle put to work should be automatically regulated so that it quickly relaxes as well, otherwise your body could, so to speak, be driving without brakes.

No reflex at all? There’s a short between the nerves from that muscle to your spinal cord. The Harpo Marx over-reaction (e.g. too much reflex)? Then the programming in your spinal cord itself that regulates reflexes is out of whack and you could have what docs call “upper motor neuron lesions.” Try playing pickup basketball with those.

A final overall objective is to see whether or not the lines of communication from the far reaches of your physique to your brain are in working order. Your brain should be getting information about the event as well, so in addition to the little automatic kick and bounce-back, you’ll feel the tap and the movement in your quadricep, hamstring, and all the supporting tendons and ligaments there to make it happen.

In grading your tomahawk test, doctors will give you a number between zero and four. Zero means no reflexes, four means repeated contractions, which is that unwanted over-reaction. Getting a two or so means you have a healthy spring in your step.

And if the good doctor forgets his little rubber toma—okay, okay, it’s called a plexor, a PLEXOR! Or simply a “reflex hammer”. Happy? You win bar trivia night. Now, where were we? Right. If he forgets it, no worries. They can use the side of their stethoscope or even a soft chop with their hand.

But don’t tell that to the plexor manufacturing companies.

(Don't visit the doc without doing your homework. Here's how to Prepare For Your Next Doctor's Appointment.)