the goal set out by the researcher jay alberts is to reach a certain cadence on the bike, 80-90 rpm for at least 30minutes at a certain aerobic rate, i think 70% of your maximum rate determined by someone who knows how to determine that. you can achieve that on your own at home or at the gym. those that can't do it on their own can be "forced" to do it on a stationary tandem bike by a stronger partner, at least the cadence part. early research used rats and primates which of course have to be "forced" to exercise. a better term might be assisted exercise.

the goal is to exercise and stay in shape, which everyone should do with or without pd, research on exercise's benefits has been going on way before this tandem bike research came out, in animals vigorous forced exercise has been shown to be neuroprotective, in humans there is a benefit for some people that lasts for awhile after the exercise is stopped based on a few trials.

google PEDALING FOR PARKINSON'S. I might be off in describing the exact methods that program uses. personally, in the early years after diagnosis i played men's soccer year round for 4 years and after a game my symptoms were diminished for at least a few hours, can't say if it slowed my progression.