This is my first post so please be patient.
1. Sys A,B operate at 3000 psi. Stby operates at 2900 psi as far as I know.
2. With the installation of the RPR (Rudder Pressure Reducer) Sys A pressure to rudder PCU is reduced to 1400 psi above 700 ft. The RPR valve is monitored through the Sys A LOP light which would illuminate below 700 ft while descending if there was a failure of the RPR.
3. Therefore the step as given by Boeing are quite logical to me:
a. Disengage the Autopilot (Process of Elimination)
b. Switch the Yaw Damper Off (Process of Elimination and it would trip off anyway when you select Sys B FCS to Stby Rudder in the next step)
c. Select Sys B FCS to Stby Rudder. This closes the Sys B Primary Flight Control Valve, whereby the rudder PCU is now receiving pressure from Stby at 2900 psi and Sys A at 1400 psi due RPR. Therefore Sys A pressure will be overrided.
Hope this helps. But I welcome your comments or suggestions.
Bye