Mach trim operates

Explanation from PART66Online

Mach trim corrects the nose-down pitch change, or tuck-under, that develops when the centre of pressure moves aft at high Mach number. Pitch is the longitudinal sense, so Mach trim is a longitudinal trim function and assists longitudinal stability. Take care with the axis names, which are the usual trap here: pitching is a rotation about the lateral axis even though it is called longitudinal stability, and rolling is a rotation about the longitudinal axis. Mach trim has no roll function, and Dutch roll is damped by the yaw damper, not by Mach trim.