A good way to understand tennis better is to think about each shot as a combination of two actions. One is the force, power or the "gas" and the second action is the friction or the control. For example: Your normal forehand could be an 80-20. What does that mean? Eighty percent of that shot would be your desire to strike the ball or drive it. Twenty percent of the shot would be the topspin, the brushing up or the control of the ball. This shot would be used when you are in control of your position and you are trying to control the offense.
 
A 20-80 shot would be more of a defensive shot. Hit it to give yourself more time. For example: You are driven to the corner and you need to hit a shot, which gives you time to get back to the middle. This shot would be hit heavy on the second number, the control number.
 
Of course, most players think the pros are always "killing" the ball. However, they are constantly making adjustments to deal with their position, their opponent's position, the elements (wind, sun, etc.) and the type of shot they are receiving.
 
Remember, these two actions can be used in every stroke. The most important of the two actions is the second number - the control. On most groundstrokes, the second number is an "up" or topspin action. On most volleys the second number is a "downward" action or under spin. On most serves the second number is an "up" or topspin action.
 
A good example of how this works is when you receive an 80-20 drive from your opponent and you return it with a 20-80. The ball you receive is already coming fast. You do not need more power. You need more control.
 
This action can really improve your serve, especially your second serve. This action allows you to keep your racket head moving fast, but not always in the driving position. It allows the second number on the shot to have a fast racket head speed with the purpose for controlling (make a safer shot) the ball. Losing your racket head speed is one of the most common mistakes in tennis. This is a good way to understand your game and make constant corrections.
 
Good luck on the Court!