What is the primary effect of extremely short rise or fall time on a CW signal?

When a continuous wave (CW) transmitter is keyed on and off, the transition creates a waveform envelope. If the rise time (turning on) or fall time (turning off) is extremely short, it acts like a harsh square wave, which mathematically contains a wide spectrum of instantaneous frequencies. This rapid transition produces unwanted broadband sidebands commonly known as 'key clicks.' These clicks extend far beyond the necessary bandwidth for CW and cause annoying popping interference to nearby stations on adjacent frequencies.