well.. the free-wheel thing is probably the most likely.. but there could be another reason.. From what i gather, this bike has a little starting clutch near the variator.
When you pedal at some minimum speed this start-clutch's pads expand outward by centrifugal force. They then contact and spin a drum that's rigidly attached to the crankshaft... then the piston moves up and down and starts the engine by pedal-power. Stop pedaling and the clutch pads retract (spring return). The engine continues to run. Whenever the engine runs that drum is always spinning.
Now if the start clutch's pads are for some reason dragging on that drum, they will force the pedals to turn.. this action is just like when starting but the forces are reversed. The engine could spin the pedals instead of the pedals turning the engine.
The cause of the pads dragging could be rusty clutch-pad pivot joints or maybe it's internal springs are weak or broken.
"THIS":http://www.mopedriders.org/article_view.php?faq=2&fldAuto=15 moby manual lists lubricating some clutch (the variator unit?) with a grease gun / grease nipple.. as a normal maintainance procedure (Page 17). So, I'd take a look at things and lube everything that should be lubed.