Mach tuck or mach tuck under is the phenomenon whereby some aircraft tend to dive suddenly as they accelerate through the transonic speed range.
When airflow over the wings reaches the local speed of sound, shock waves form. Air flowing through these shock waves experiences a sudden increase in static pressure. This sudden pressure increase causes and equally sudden deceleration which tends to cause the airflow to separate from the wing just behind the shockwave. The overall effect is a loss of lift and an increase in drag, similar to that caused by ordinary low speed aerodynamic stall. For this reason the effect is called shock stall.
The shock waves form wherever the airflow reaches the local speed of sound, so they form first at the wing roots where the thicker aerofoil section produces the greatest acceleration. This means that the shock stall affects the wing root area before it affects the wing tips. In swept back wings, the wing roots are ahead of the wing tips, so the loss of lift due to shock stall at the wing roots causes the centre of pressure to move rearwards. this causes the aircraft to pitch nose down in the mach tuck under.
With straight wings the effect is slightly different. Shock stall reduces lift behind the shock waves so the rear area of the wing is most affected. This means that the C of P initially moves forward, causing a tendency to pitch nose up. But as speed increases further, the shock waves move rearwards causing the the C of P of the wings to move rearwards to about the 50% chord point. It is this rearward movement of the C of P which causes straight wings to tuck under.
The overall effect in both cases is that the aircraft tends to pitch nose down as it accelerates through the transonic speed range.
The mach trim system counteracts this tendency by adjusting the longitudinal trim of the aircraft.