O/D switch ONLY changes engine braking
 

2003 RX300 AWD with u140f transmission:
I have OBD2 code P0766 (S4 SOLENOID STUCK OFF), and predictably no 4th gear, but what I cannot figure out is why, when I press the O/D button, it does NOT do nothing...instead, it engages/disengages ENGINE BRAKING.
My understanding is that the one-way clutches in the transmission are NOT externally controllable, and yet for some mysterious reason I am able to engage/disengage engine braking using my O/D button. How is this possible?

[As far as the P0766 code, I have verified that the S4 solenoid both ohms out correctly and click tests correctly and that the ECM is applying 12 volts to S4 at the proper speed when the O/D button is "off" (and not when the O/D button is "on"), and so I am assuming that probably the valve body has a clog that is preventing the shift to 4th gear. I have changed the filter and fluid. I can live with that problem, but I am curious why I can control engine braking using my O/D button?].
Update: Someone else had the exact same symptoms, and after a new S4, valve body, and ECU, the problem persisted.