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They have a ball bearing where the outer hub spins. I believe the outer hub has a small weight on one of the spokes to let it spin around.
I see them everyday around here. They are very expensive things to buy! Often times, the wheels are worth more than the car.
They also have spinning hubcaps!
Friction from the bearing or as said above, a small weight, causes them to start spinning as the car driving. When it stop, inertia keeps them spinning.
Dammit... I hate this bastard server haha I had a beautiful explanation all written up yesterday and posted then clicked sumbit and it didn't go through.
As already said... now... They are connected to the rim via a bearing. Internal friction of the bearing allows torque to be transferred to the spinner cause it to spin. When the car stops the spinner has stored kinetic energy and continues to spin.