BB,
Three things that he could be talking about:
1. Mach tuck
2. Mach number variation with change in tempature, with can induce Mach tuck as well.
3. Transonic drag rise (also known as shockwaves) as the Mach number tends towards @ .90 (this can vary with the airfoil's critical Mach number)
Just a guess, but if Davies just says 'Mach number effect' he's being pretty vague!
Nial