This system description is from the 737NG FCOM.
It explains the control column (under-floor) cutout switches which have been modified in the MAX to prevent them from disabling MCAS stab trim movement. The “hidden switch” on the centre console (actually called the aisle stand) is in plain view. Its a black cover-guarded switch called the STAB TRIM OVERRIDE that will override the underfloor cutout switch. It is there to restore normal trim operation if one of those switches has failed. I don’t know if it is still fitted to the Max, but that would be moot in any event.
Stabilizer Trim
Stabilizer trim switches on each control wheel actuate the electric trim motor
through the main electric stabilizer trim circuit when the airplane is flown
manually. With the autopilot engaged, stabilizer trim is accomplished through the
autopilot stabilizer trim circuit. The main electric and autopilot stabilizer trim
have two speed modes: high speed with flaps extended and low speed with flaps
retracted. If the autopilot is engaged, actuating either pair of stabilizer trim
switches automatically disengages the autopilot. The stabilizer trim wheels rotate
whenever electric stabilizer trim is actuated.
The STAB TRIM MAIN ELECT cutout switch and the STAB TRIM
AUTOPILOT cutout switch, located on the control stand, are provided to allow
the autopilot or main electric trim inputs to be disconnected from the stabilizer
trim motor.
Control column actuated stabilizer trim cutout switches stop operation of the main
electric and autopilot trim when the control column movement opposes trim
direction. When the STAB TRIM override switch is positioned to OVERRIDE,
electric trim can be used regardless of control column position.
Manual stabilizer control is accomplished through cables which allow the pilot to
position the stabilizer by rotating the stabilizer trim wheels. The stabilizer is held
in position by two independent brake systems. Manual rotation of the trim wheels
can be used to override autopilot or main electric trim. The effort required to
manually rotate the stabilizer trim wheels may be higher under certain flight
conditions. Grasping the stabilizer trim wheel will stop stabilizer motion