Why would the propeller of a lightsport plane or ultralight w/ Rotax engine be connected to the engine through a clutch, and not turn at low rpm?
Centrifugal clutches are mainly used to provide a compliant connection between the engine and prop to absorb power pulse peaks, as well as allow the engine to be automatically disconnected from the drive system at idle.  

The big bugaboo of reduction drives over the years has always been torsional resonance resulting from the flywheel - the prop - being driven through a springy drive path by strong pulsating power impulses.  At RPMs where the pulse/feedback frequency matches the natural frequency of the drive system/prop, you get massive resonant torsional vibration, which at minimum is unpleasant and at maximum can trash the gear train.

Over years various attempts have been made to tame this phenomenon in a practical way.  It's not actually a good idea to use a centrifugal clutch in a normal aircraft.  If the engine quits, the prop can freewheel and this turns it into a very powerful airbrake. It's also a reliability issue, being a friction device in a critical power path. 

That said, a coupling that has been used with some success, and even certified at one time, was the Flexidyne Dry Fluid Coupling, which works like a centrifugal clutch but uses steel shot encasing a wavy plate in the housing to act a bit like a fluid torque converter.  The FAA certified Molt Aerocar used a Flexidyne to connect the Lycoming to the prop down a long drive shaft (which suffers from the same kind of torsional resonance problems).  Problem is, Flexidynes get really hot, and you still have the windmilling problem (Molt Taylor used the Flexidyne in several of his homebuilt concepts but he never really addressed the freewheeling prop issue) and a propeller brake might be in order if you want to be able to glide (or a feathering or folding prop).

You see centrifugal clutches used in paramotors, mostly because it allows the motor to be running at idle without the prop turning, a handy feature for a paramotor.  I wouldn't be surprised if the Dragonfly's engine is actually using a Flexidyne instead of a friction clutch.

The most common compliant coupling between motor and prop however, is rubber.  Most commonly a rubber drive belt system, used everywhere on ultralights and some automotive aircraft conversions.  Or as successfully used by Rotax, a rubber coupling in the gearbox itself. Rotax, after a lot of effort, was able to create a rubber drive isolator that would be effective in a given engine's operating range.  This would have taken a massive engineering effort over the years to perfect.