As you open the sheave the fan speed slows. Closing the sheave or V groove makes the fan turn quicker, pushing the belt to the top of the V groove acting as a larger diameter pulley. Make sure your belt is within the right length for this pulley adjustment. You'll still need to set the final tension on the belt when you get the right sheave speed.

I learned something today. thank you my professor at YTU (KZfaq University)

You also need to make sure the pulleys are dead on parallel to each other.

What does a sheave do for power? Torque? Nothing I imagine right it's just to fit larger belts?

at what point is the adjustable sheave considered fully open?

First! Turn Power Off