The moves in salsa are created through a pulling tension between the leader and the follower. The leader has to ‘pull’ the follower, which creates tension between them, and then the tension is used to create the moves themselves. Without any tensions there would be no moves, therefore there would be no dance.
In the beginning phase of the dance the leader is concerned about recalling all the moves he has learned in class, and about keeping up with the music. The follower in the beginning is concerned about knowing what each signal that the leader is sending means, how to follow and react to those signals, and at the same time trying to keep up with the music. Once those things start feeling more natural and they are less of a factor, then the dancing experience starts to grow and become richer. Intermediate leaders and followers are able to recall/follow moves and keep the timing of the music. In order for intermediate dancers to go up to the advanced level they must master tension.
Tension is the vehicle through which the moves happen. An analogy I use is the old car versus the new car. If you have ever driven old cars, you know that you really have to press the gas pedal in order for the car to accelerate, whereas the new car, you barely touch the gas pedal and the car accelerates. The old car is not as efficient and effective as the newer car; therefore more energy is required in order to get the old car to accelerate.
In dancing (or any physical activity for that matter) the more effective and efficient you become, the less tension (energy) it is required to achieve the same move (result). Advanced dancers use less tension and energy to create the same or better moves than the moves of intermediate dancers which they do using more tension and energy.
The end result is that you can have better quality dances because tension is the art of leading and following. The more you master tension in dance the more effective and efficient you will be at leading and following. Also, you will be less tired if you are using less energy throughout the dance then you are able to have more dances!