Before I get into how to use it, I have to explain its components.
A tempo is the speed at which an individual does an exercise, and it consists of 4 components, Eccentric movements, Pause, Concentric movement, Pause;
It comes in this format, 0-0-0-0, that would indicate a movement that is instantaneous, which is impossible, so a more realistic Tempo is this, 1-0-1-0 which would mean the eccentric and concentric movements would only take 1 second to do.
Studies have shown that tempo can increase the intensity of an exercise or improve specific aspects of such goals are desired by tweaking the numbers that I have mentioned.
So if an athlete is training for power, a tempo would look something like this, x-x-x-x, which means do it as soon as possible basically, which develops power;
But if your goal is muscle hypertrophy, and muscle strength & stability, a tempo would look something like this, 4-1-4-2, so that means the movement would be so much slower than how a normal person would do the resistance training.
You might ask why specifically 20 reps, why not more? Now that is a discussion for another day.