With a 'normal' AOD valve body, there are only three gear positions on the shifter, yet four gears counting Overdrive. The Selector is P-R-N-OD-D-2. Normally that isn't a problem. The AOD will shift itself at WOT 1-2 at 4500rpm, and 2-3 at 4200rpm. The 'high speed governor' in a Mustang will shift about 300rpm higher. To manually control the shift at higher-than-normal RPMs, you start with the gear selector in the lowest position (usually marked '2') to manually hold it in First, then when you reach the RPMs you want to shift you briefly move the shifter to 'D' then immediately pull it back down into '2' for Second gear. Then when you reach the RPMs you want to shift again, you move the selector into 'D' for Third (or Drive). My 393Ws best ETs are found by shifting around 5500rpm. If I leave it in D, the car will shift itself at 4500/4200 and it runs a full second slower.
Anyway, the Silverfox SPT-series valvebody is custom-calibrated to shift at the RPMs you specify when ordering. It is adjustable with a setscrew to -200 and +600 of the 'customer-specified' shift point so you can tweak it a bit. I went with the SPT-R that allows the normal 1-2-3 shift pattern (without the 2-D-2 shuffle) and OD is an electronic button.