Mach Tuck
Everything A375....stated is correct. And also the Wikipedia has the aerodynamics.
The shock wave desensitizing the tailplane and eliminating the download on the tailplane it was inevitable causing a dive to become steeper.
This characteristic gave rise to the high set tail planes ...out of the turbulent (stalled) flow and produced the design described as "Mach Trim".
Mach Trim was a built in program that altered the tailplane automatically to provide greater tailplane (Tail plane plus elevator) power and provide conventional reaction to control inputs.
These days it is all done by computers sensing speed, altitude, temperature control input, pilot alcoholic content, colour of eyes, bald or haired.........LOL.