The classic feature of Parkinson's disease is the pill-rolling tremor at rest, which refers to a fine, rhythmic movement of the thumb and fingers resembling the motion of rolling a small object. This tremor is most prominent when the patient is at rest and typically decreases with voluntary movement. It results from the degeneration of dopaminergic neurons in the substantia nigra, leading to altered basal ganglia circuitry. Unlike intention tremor or choreaform movements, the resting tremor is distinctive for Parkinson’s disease.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson et al., 20th Edition.