This video demonstrates how an overrunning pulley works. Start watching at time 1:15. Use caution and do not scratch internal wiring if you elect to use a screwdriver to hold a cooling fin. I choose to quickly spin the pulley with a flick of the hand rather than poking tools into an alternator. The pulley locks up in one direction and free wheels in the other.
https://www.youtube.com/watch?v=-0TOVgXUrw8
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Alternator Decoupler Pulleys | Toyota Nation Forum