The governor modes in the CC software are a bit "iffy" -- especially the "latest" software. Their operation is dependent to a great extent on the brand and type of motor you choose. Some work better than others. Some motors and software revisions give you erratic operation and the governed motor doesn't maintain a steady speed.
The governor modes in Version 1.14 so far seem to work the best. The remainder of the 1.14 SW seems pretty good, too.
The choice of High or Low is based on the number of poles your motor has -- and in the end, you choose one or the other to get the head speed you're looking for. If you choose Low and just can't get the head speed, then you choose High.
With the Align 420, you'll most likely end up using High governor mode.
Once you select the governor mode, you simply dial the head-speed you're looking to maintain into your throttle curves. If you find that you're getting the desired head speed with the throttle curve set to 85%, then you simply dial in a flat line throttle curve with 85's all the way across. If the head speed you're looking for happens at 90%, the you dial in 90's all the way across.
If you choose fixed mode, set your ATV (throttle endpoint) for full throttle so that the red LED comes on solid, add about 5% so it'll stay on at full throttle, and you're set to go.
Don't forget to set the throttle low endpoint so you can plug in the battery without the darn thing firing up and doing the funky chicken. I usually set my TX up so that I have to hit the Throttle hold switch to arm the ESC. I plug in the battery with low throttle, high trim, and the throttle hold switch OFF. To arm the ESC, I cycle the HOLD switch. When the hold switch goes back to OFF from the ON position, the ESC is ready to rock and roll.
From that point on, you dial in the normal throttle curves for the different flight modes -- normal/hovering, idle up 1, idle up 2. The T-Rex manual has a pretty good table of recommended throttle and pitch curve settings printed in it. Start with those if you use fixed mode, then experiment from there.