Originally Posted by
MurphyWasRight
That switch existed on 737 until max, one of the two cutout switches disabled all automatic trim the other disabled all, including manual, electric trim.
This was discussed in the prior threads but no solid reason for change in MAX where either switch disables all electric trim.
Also brought up is that the training changed some time ago to allways use both switches on suspect trim runaway.
Had this changed not been made the ET crew might have been able to recover by enabling manual electric trim only.
It might not have been required if the ET crew trimmed to neutral or even nose up before shutting off the trim. Recall that the electric trim was re-enabled at the point you are thinking of, but they did not retrim the airplane using electric trim; instead they tapped the button a couple of times without making a change to the stab trim and then attempted to re-enable the autopilot.
I suspect the change was made in order to prevent confused pilots shutting off the working trim system and allowing the erroneous one to keep working, much like pilots have shut down the remaining functioning engine in a panic.