what is it for, and what does it do?

Rotates the player

ahh, simple enough, how does it work?

Works with degree. So 180 equals a half spin.

how many different positions could you change the player, or uke?

Well with Engagerotation you can either make Tori and Uke face each other or have them back to back. Everything except 180 seems to bug because the joints and bodyparts don't apply correctly.
In Engageplayerrotation you have 0,0,0,0,0,0
each digit refers to degrees. x-Tori, y-Tori, z-Tori, x-Uke, y-Uke, z-Uke.
So you can flip around the x, y, z axis (I'm not totally sure because it seems to not work properly anyway)
Never tested it but I guess everything except 0, 180 and 360 will glitch.
A modder might be more experienced with this.

ahh, thanks for the help.