[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 5-48]

After Mach buffet happened, the aircraft will either get nose-up or nose-down pitching moment due to movement of CP.

“Mach tuck” (= “Tuck under“)  is due to the CP moving aft – a diving (nose-down) moment is produced;

In contrast, if CP moving forward – a nose-up pitching moment is produced.