On is used to describe the period when the patient's medication is working. Movements are normal or almost normal. The patient is in a relative state of good motor function. Such on time may or may not be associated with dyskinesias. Tremor most commonly occurs when the muscle is at rest.
Tremors usually disappear during sleep and increase under stress.
A person may well be on and still have some tremor. On is when the medication is working, not necessarily an absence of tremor. Dyskinesias are abnormal, involuntary movements of the voluntary muscles, e.g. in the arm, leg, hand, etc. They can vary greatly in pattern from barely discernible twitches and jerks to twisting, writhing movements involving almost the entire body. Please note that dyskinesias are not the hard tremor of the disease.