A rhythmic jerking movement of the patella ("kneecap" or the small bone that is in the front of the knee) produced by grasping it between the thumb and the forefinger and pushing it suddenly and forcibly toward the foot.

This is an abnormal reflex with alternate contractions and relaxations of the quadriceps muscle (great extensor muscle of the knee, forming a large fleshy mass which covers the front and sides of the femur or thigh bone).