High speed when you want to go home and get laid, low speed when you want to hook up with your customer.
Ok, seriously.......just think of it the same way you would your wand. A lower speed allows deeper water penetration for a deeper clean on thicker carpet. Use low speed near transitions so you don't scratch tile.
I'd strongly recommend pre wanding all edges before you start cleaning with the t-Rex.
Also, since the regular head slobbers so bad when you lift it off the carpet, it helps to keep the reservoir head near the area you stop at.
This isn't trex specific. But I just got my rotovac at the end of last year. I have been wanding edges after using the rotary, and wanding the whole room with a dry pass as well. I am not a fan of the swirl marks which is mostly the reason for wanding, but an extra dry pass doesn't hurt