a mach trimmer works by pulling back on the stick.
on the DC9 series, it is called a Mach Trim Compensator. AS you go faster the nose wants to drop, so a nifty gadget pulls back on the stick so you don't have to.
you test it prior to flight. if it fails, you have to fly slower.
there is a pretty nifty little rod that comes up to show you how much p;ull up is being used.