A short review:

the overdrive uses an oil pump that is constantly pumping oil through a valve and applying oil pressure to the pistons that pull the brake ring for planetary gear engagement against a fixed brake cone.

When you engage overdrive you are actually stopping an escape path for oil flow that works on a piston in the lower regulator valve. This piston going up in its bore makes a spring get tensioned and the regulator valve go from its resting low of 15 psi to over 400 psi and the OD apply pistons go up in their bores and the brake ring goes against its cone and this stops the planet sun gear and OD is now working.

If the regulator valve sticks you get OD when you don't want it. This can be caused trash blocking the relief port for regulator valve, stuck reg. valve., or stuck reg valve that lets pressure go so high that the bars the pistons press up against get bent and pistons can overtravel and get stuck out of their bores. The pistons getting stuck won't let the brake off which gets you back into direct (out of OD).

Overdrives I understand, Allen Greenspan I don't.