It pertains only to those amplifiers using vacuum tubes AND a standby switch. They plug in their guitar, turn on their amplifier, and start playing without regard to what is happening inside of the amplifier. But if it is a higher-powered amplifier such as a Fender Twin Reverb, Deluxe Reverb, Vibrasonic, Bassman, or Marshall, in many cases it is using voltages that exceed the vacuum tube manufacturer's ratings, and precautions need to be taken to ensure long tube life. The standby switch is used to turn off the high voltage to the amplifier's tubes. The high voltage causes current to flow in the vacuum tubes.
Current flow is the movement of negatively charged electrons to a more positive place.
The vacuum tube emits the electrons from a cathode and they are collected by a more positive anode or plate contained within the vacuum. If a voltage more positive than the cathode is not present on the anode, the negatively charged electrons emitted from a heated cathode will not be attracted to the anode and, therefore, there is no current flow and no amplification. A small voltage less positive than the operating voltage on the anode is used to heat the cathode. In most cases, it takes from 45 to 60 seconds to reach the proper operating temperature for the cathode. It is during this warm-up time that damage to the vacuum tubes occurs when the high voltage is applied and it peaks simultaneously.