b737ng tec q.
hello justin,
for the mach trim, you are correct that the elevator is moved slightly up & so controlwheel will move aft a little bit as well. as you say, this creates a force for a nose up trim input as in every other flying machine. now in the boeing & other aeroplanes with powered flight controls, this "force" is generated by "Q"feel(pitot pressure from pitot tubes on vertical fin),fed to "Q" feel computer,which uses hydraulic pressure to compress a calibrated spring. to get rid of this "spring" pressure, either the pilot or the autopilot, when engaged, electrically moves the stabilizer , which in turn, by mechanical interconnections to the spring mechanism, also called "the neutral shift" system, extends the spring back to neutral.
in case of mach trim operation however, this spring force is eliminated by moving the neutral shift mechanism altogether without any stab movement.
to summarize: pilot or autopilot controlwheel inputs need stab trim action
mach trim input moves controlwheel(aft) & elevator(up), but then moves neutral shift mechanism & not the stabilizer