Overmodulation ($m>1$)

For this single-tone AM model, overmodulation means $m>1$. The signed factor $q(t)$ becomes negative whenever $\cos (\Omega t)<-1/m$. On such intervals,The sign reversal is a $\pi$ radian shift of the carrier's wave phase. At each zero of $q$, the wave envelope vanishes and that phase is undefined. The waveform itself is continuous: the formula has not clipped it or created extra spectral lines.

At $m=1.4=7/5$, sign changes occur at message phases $\Omega t=\arccos (-5/7)$ and $2\pi -\arccos (-5/7)$ within each cycle. At the trough $\Omega t=\pi$, the desired signed factor is $-0.4$, whereas its nonnegative wave envelope is $0.4A$. Ideal envelope recovery followed by subtraction of $A$ would give $-0.6A$ there, rather than the desired message voltage $-1.4A$. This is loss of polarity in the detector, not loss of the message from the full modulated signal. Actual transmitter clipping is a separate signal distortion that can generate additional spectral components.