Shaking the body of a parkie relieves symptoms.

Earliest writen reports of this comes from the "father" of modern neurology, french Dr. Jean Martin Charcot, who was alerted to this by PD patients who came to see him in Paris on very bumpy train rides of those days. He tried to replicate the idea by making a wooden chair mounted on some mechanical parts an actioned by a manual lever to simulate the reported bumpy ride. He did not do much with it and abandoned the idea.

The same effect, but much more pronounced, was modernly picked up, again by chance, by a cardiologist (alerted by a heart patient, coincidentally a parkie) performing external counterpulsation,ECP, or EECP, and FDA approved method for heart angina, which involves a lot of rhythmic shaking of the body, which literally jumps up from the treatment bed with every impulse of the machine. So amazing were the effects of ECP/EECP on PD that the cardiologist carried out a study with several dozen parkies from all over. Unfortunately the study resuls never saw light of day due to the sad and sudden departure of the researcher.

Nevertheless, the idea is not entirely new, having been pursued by PD expert, Dr. Lieberman, which visited the cardiologist who found the PD/ECP/EECP connection and compared it with a similar, very simple technique, named "acceleration therapeutics", developed by Dr. Marvin Sackner with a sliding bed that moves back and forth at a rather rapid frequency, imparting the body's fluids and innards with a good shaking, resulting in the rather amazing, albeit temporary, relief of PD symptoms.
Likewise, Ed.Phillips, a heart patient and inventor, modified one of his machines to make it effective in the relief of PD symptoms and came up with something called "blood pressure modulation therapy".

Releaving of (for example) a crampy leg by shaking it vigorously by hand by a PT, has been a trick of the trade, long known and used a in many sports, for example, soccer, where it is common to tend to a player's crampy leg by shaking it right on the field sidelines, with the player laying on the grass,in full view.

Finally, there are scientific and intuitive explanations on how and why exercise and shaking (a form of passive exercise for those unable to move by themselves) relieves PD symptoms in several articles by Dr. Lieberman.

If any parkie wants to get a taste of the effect of shaking on PD symptoms, next time you go "off", i.e., get stiff, tremoring, slow, bent, etc., ask a helper to grab both your feet by the ankles, while you lay on your back and then give both your legs, simultaneously, a vigourous up and down shaking, as if trying to send vertical waves along a horizontally extended piece of rope. Another effective way is to have the distressed parkie kneel down on the floor, perhaps resting his head on a bed and a helper grabs and lifts his hand as high as possible and then shakes them together, trying to send waves along his arms which end on his neck. If the helper gets the rythm just right, i.e., frequency and intensity, he will notice beyond doubt the sudden relaxation of limbs which were lead-stiff just a moment before and you, the patient, will/ may be gratefully surprised at the relief experienced from such uncomfortably excruciating symptoms and may begin to wonder what comes into play to afford such relief, which appears to be a combination of the cutting off momentarily of the electrical nerve signals that have you all contorted and a simultaneous restoration of blood flow to the brain caused by stiff neck muscles. Unfortunately, this shaking is very tyring on any helper, and can only be sustained for no more than a couple of minutes at a time, upon which, symptoms return rather quickly. This is what probably discouraged Dr. Charcot early observations. However, with modern resources, i.e., electric motors, gizmos and gadgets, it is possible to impart sustained, precise and prolonged shaking, only limited by the patient's patience, with relief of symptoms now being sustained over longer periods of time, in a way making them permanent if therapy is kept. Now, that is something to ponder and instead of trying to find out the exact mechanism at play, I would make the technique available to the millions who could benefit from it right now. Or, you could borrow Teresa's LandRover.