What is mach tuck?

Shockwave forms over the wing at the point where the airflow reaches sonic speed at first. This results in reduction in total lift produced by the wing. To avoid tip stalling (thereby keeping the effectivity of the ailerons) wing designs incorporate higher camber towards the root resulting in roots stalling first. The outcome is shifting of the Center of Pressure (CP) towards the tip. For high speed aircrafts' swept wing this outward shifting of the CP also means rearward shifting of the CP.

The rearward CP movement results in pitch down moment of the aircraft which is known as Mach Tuck. Mach Trim functioning at specific Mach No. for specific aircraft types (0.7 M for Airbus 310 for example) gives a Nose Up Trim order to counteract the Mach Tuck. Understandably, this is imperative that the Mach Trim works at all times like AP in CMD or CWS or DISENGAED mode.