According to Wikipedia,
The Jendrassik maneuver is a medical maneuver wherein the patient clenches the teeth, flexes both sets of fingers into a hook-like form and interlocks those sets of fingers together.
It is generally used in sluggish knee jerk reflexes where you'd expect better ones. This method somehow elicits a better reflex. It is mysterious to me as to how clenching of the teeth and the like, can have an effect on the knee jerk. Ganong's Physiology says that the maneuver increases the gamma motor neuron discharge. But could someone explain in detail of how this happens?