Modern Toyota alternators have what’s called a decoupler pulley on the alternator. They allow the alternator to spin faster than the engine. This prevents a squeaking noise on upshifts. They are known to fail and freewheel in both directions, or completely come apart. The failure seems to happen more frequently with time spent on the rev limiter. I personally have had 3 fail.

If the pulley has more ribs than your belt, use the inside ribs. The pulley is larger than stock, otherwise known as an underdrive pulley. This does reduce wear on the alterator at high RPM, but it’s doubtful you’ll see any horsepower gain.