Originally Posted by
ScepticalOptomist
You would use them if the trim switches on the control column / yoke were inoperative. If you found you couldn’t trim the normal way, you could use the alternate trim switches instead.
Well, but as I understand there is a "Column cutout" feature that would disable the stabilizer if it senses a failed or jammed trim switches (either from the primary or alternate trim switches). So, I guess this means if the trim switches on the yoke fails, the Column Cutout feature will shutdown the stabilizer trim system and prevent trimming from the alternate trim switches. The flight control system will also revert to Direct mode which will even prevent pitch trim through the elevators.
Column Cutout
The column cutout function is designed to stop the effects of uncommanded pitch trim input from jammed or failed pitch trim switches. If a nose up or nose down pitch trim command from either the primary or alternate trim switches is opposed by either control column for more than 2 seconds, the column cutout function disables the specific pitch trim switch commands until the switch input indicates no trim input. If the column cutout function remains active for more than 20 seconds, i.e. the trim switch remains jammed or failed, the STABILIZER L2/R2 EICAS advisory is displayed, and the uncommanded input is removed by system shutdown.