When an amplifier of a fixed output impedance (50 ohms) is tuned for minimum VSWR into a load (your antenna), then maximum power is transferred from that device to be radiated into the ionosphere.
Let's assume that our RF amplifier feeding our antenna (and in the interest of simplification is 100% efficient) has 5 volts applied to it, and the current is 20 amps. 5 * 20, that's 100 watts.
The SWR is 1:1, so in this ideal amplifier there are 100 watts delivered to the antenna, and the RF power amplifier transistor doesn't get hot.
(But, of course, in the real world, there is no such thing as an RF amplifier device (either transistor or vacuum tube) that is 100% efficient. Therefore, those devices will get warm to some degree, even when perfectly matched at a 1:1 VSWR.)
Now, suppose that we deliberately change things and mistune our antenna so that its feedpoint impedance is lower or higher than 50 ohms.
- Less power will be radiated by the antenna.
- The current through the output transistor may change up or down slightly, but now more power will be dissipated in the transistor, making it hotter.
- As we continue to detune our antenna, even less power will make it to the antenna and more power will be dissipated in the transistor. Eventually, so much power will heat up the device to the point where it is destroyed.
Higher SWR = less power to the antenna to be radiated into space = more power to heat up and destroy our device .