Does it simply slow down because the chain is slowing down and it is therefore slowing it down or is there an electric signal that is sent to the engine telling it so stop or WHAT??
There is a magical part on scooters and motorcycles that's called a clutch. The clutch engages at a certain rpm(on scooters mainly), so when you twist the throttle, the clutch engages and drives the rear tire. When you slow down, the clutch disengages, allowing the engine to freewheel and slow itself down, as long as the throttle is at the idle position. On motorcycles with manual clutches, when you pull in the clutch lever, it disengages, and when the lever is out, the clutch is engaged, allowing the engine to drive the rear wheel. The same principle applies here too; as long as the throttle is at the idle position, the engine will slow itself down. The engine slows down because the throttle is closed, not allowing a large amount of air/fuel mixture into the engine.
IT SLOWS DOWN BECAUSE THE CHAIN IS SLOWING DOWN
When you release the throttle (right hand grip), the carburetor slows down how much fuel is going to the engine. No fuel, no run - the engine slows down. Same as a car. Take your foot off of the accelerator - less gas goes through the carb into the engine - the engine slows down - when the engine runs slower, the vehicle slows down
