I think I get it now...
When I torque the U-joint it will put pressure on the u-joint spacer(#17) which is in contact with the speedo gear (#16) which is in contact with the speedo gear spacer (#15) which is in contact with the inner race of the main shaft rear bearing (#13). The speedo gear becomes "sandwiched" and therefor remains locked to the main shaft.
Do I have that right?
