I hope you have better things to do on a Thanksgiving. But I wish you a great one.
On the right side of the transmission below the motor you will see the ROTAC. This is a air driven device with orifices in the air lines, that moves two plates (top and bottom) that hold the gear assemblies. The top plate has teeth along its perimeter to mate with the ROTAC. The rotac at its top has 2 magnetic switches that confirm high or low gear. They should light up. Below this is the shot pin. It is spring loaded with a air cylinder that locks the bottom plate in high or low position. There are holes drilled in the plate for this. There is a switch that verifies the fact that the shot pin is in the plate or out.
I have with the help of a assistant manually changed gears. I fire the shot pin to retract it. I fire high or low gear to change gears while the assisitant moves the spindle back and forth. You will feel the gear change. They the shot pin hlds it there.
The way the machine does it is very similar but every step has switch verification.
The machine will not command the ROTAC unless it sees the shot pin retract.