The governor pressure developed acts on the end of the 1-2 shift valve and as road speed increases it becomes high enough to overcome the opposing forces of the spring and throttle pressure.
The shift valve assembly moves upwards and allows line pressure to pass into the circuit to the apply side of the front band servo.
The transmission is now in second gear.
Once the 1-2 upshift has taken place, the differential area is no longer effective.
This means a 2-1 downshift cannot take place at the same road speed as the upshift since governor pressure must drop to a lower value to allow the valve to move back to the 1st gear position.
For downshifts to occur, the vehicle speed must be somewhat less than the speed at which upshifts occur.
The shift spread provided prevents the transmission from "hunting" up and down between two ratios and is referred to as shift speed "Hysteresis".
