Well...not sure if it's the same on the 767...(and I would guess that it was), but the 757 "Mach/Spd Trim" works using IAS when flaps/slats are extended, and Mach No. when they are retracted.
In both cases, as NSEU stated above, the Mach/Spd trimmer is only "trimming" (if it needs to) when you are manually flying (autopilot not engaged) and you are not electric trimming...!!
I'm off to sit in a dark room...