Why should the phase of a PSK signal be changed at the zero crossing of the RF signal?

Phase Shift Keying (PSK) modulates data by shifting the phase of the carrier wave. If a phase transition occurs abruptly at a peak in the wave's amplitude, it creates sharp, instantaneous voltage changes. These sharp transitions act like a square wave, generating high-frequency harmonics that spread the signal and widen its bandwidth. To prevent this, the phase changes are timed to occur exactly when the RF signal's voltage crosses zero. At the zero crossing, the transition is perfectly smooth, which effectively eliminates unwanted harmonics and keeps the transmitted signal's bandwidth as narrow as possible.