The wikipedia article on continuous-phase modulation states the following:
For instance, with QPSK the carrier instantaneously jumps from a sine to a cosine (i.e. a 90 degree phase shift) whenever one of the two message bits of the current symbol differs from the two message bits of the previous symbol. This discontinuity requires a relatively large percentage of the power to occur outside of the intended band (e.g., high fractional out-of-band power), leading to poor spectral efficiency.
I want to know how this is true since in M-ary PSK the amplitude is constant so isn't the power constant too?