Put your forefinger gently on the patient’s loosely opened jaw. Tap your finger gently with a tendon hammer. Explain the test to the patient or relaxation of his jaw will be impossible.
Normally, the jaw will not jerk much, unless there is a lesion.
This would cause one lower quadrant (on the effected side) to droop. It's important to note that UPPER motor neuron lesions affect ONLY the LOWER face.
This is because the part of the facial nucleus controlling the upper half of the face is bilaterally innervated.