centrifugal clutch

[autogyro]

I would guess that using two clutches with the centrifugal on the engine output (the only usable place) is to make the vehicle easier to drive by taking away the need for the rider to control direct engagement.

[flynfrog]

A sprag (clutch) is a 'one way' clutch that only allows drive in one rotational direction and freewheels in the other.
In Borg Warner automatic gearboxes you can easily instal the 'sprag' the wrong way round and lock the entire unit.
If the sprag is between the engine and the gearbox there would be no over run braking but changing manual gears would be easy if the throttle is lifted.
The unit pictured by flynfrog certainly looks like it has two clutches, one cetrifugal and one multi plated.
What unit is it?

it is out of a dirt bike. When you pull the shift lever it activates the plate clutch allowing you to change gears.

They are usually on less performance models where the rider is likely to be less experienced and doesn't want to use the hand clutch

[tommylommykins]

Is there any functional difference between a sprag and a freewheel? From what I read on wikipedia, the difference between the two is purely in how the mechanics achieve the same thing

[autogyro]

Is there any functional difference between a sprag and a freewheel? From what I read on wikipedia, the difference between the two is purely in how the mechanics achieve the same thing

One way clutch is a general term.
Sprag refers to the components that grip the freewheel surface in a specific type.