As I understand it, a jackscrew is a mechanically irreversible mechanism? So once the motor has driven the stabiliser to wherever it has been commanded, it stays there till something breaks or changes?
Its odd that this none-reversible mechanism exists only in pitch control, not the others?
N.
Well, it's only pitch trim that moves the entire airfoil in question (as opposed to just a control surface), which is necessary because of large changes in trim needed for the range of Mach numbers airliners operate at in the course of a single flight.
The jackscrew is used because the forces across the whole stabilizer can be very large and you need something where the control end (electric motor or manual trim wheel) can easily move the thing being controlled (the stabilizer), but the forces on the controlled end can't easily overpower the control end and move everything back to where it started, or beyond. A screw converts rotational motion into linear motion, so it works very well for this: you can easily move a screw into a hole by rotating it, but if you try prying it back out directly, the threads will hold it in place until something in the assembly undergoes structural failure. But if you want to remove it, it's easy to get it back out by turning it the other way.
The same goes for a jackscrew: the motor and trim wheel operate on the jackscrew by rotating it, so they can move it easily, but since the lever that moves the stabilizer is screwed into the jackscrew, forces on the stabilizer act perpendicularly to the threads and this prevents stabilizer forces from forcing the mechanism.
The problem that was encountered on the Ethiopian flight was that the elevators were acting one way on the stabilizer (the crew pulling up), the general forces in the stabilizer were acting in the opposite direction (from down), and the high airspeed meant that the forces involved were, in general, larger than had been designed for, and this caused the jackscrew to bind, which made it difficult to move the jackscrew assembly even by rotation. The motor could put out the force needed, but the crew couldn't, especially while simultaneously trying to haul back on the yoke, so they gave up trying and turned the trim motor back on so that they could use the extra power it had available, but since MCAS was still going off, it was regularly signaling the motor to pitch down.
The solution was for the crew to release the yoke, which would lessen the forces on the stabilizer and ibid the jackscrew, and dedicate their full strength to rotating the trim wheel, then to haul back on the yoke again until the nose was above the horizon, then let go of the yoke and work on trim again, and so on with that roller coaster ride until trim was in an acceptable position.