Overmodulation is by definition to condition where DOWNWARD modulation
exceeds 100%. I was describing the effect of OVERLOADING a class C
modulated stage. Trying to even approach 100% upward modulation with an
overloaded or underdriven final will of course cause modulation to
exceed 100% in the downward direction. This distortion of the
modulation characteristic is carrier shift. Nonlinearity in a
linear(AB1 amplifying AM) final would cause the same sort of problem.

I learned this the hard way, and found that designing the final for the
intended PEP output (and with MOSFETs applying a little gate modulation
or driver modulation along with the collector modulation)solves the
problem.