Frequently, when a horse is unfocused and not relaxed, people will say there is "tension." The problem with using this word in this way is it suggests that we don't want tension in our horses. When we do this, it starts to suggest that we cannot have relaxation with elasticity and suppleness if we have tension. Though it is correct that we don't want a level of mental tension that leads to a distracted or overly excited horse, we do want a certain level of muscle tension in their body to support the overall position at that moment.
To understand this more, picture a dancing ballerina. There appears to be no physical tension because she is free of mental tension and her body is not too tight. Instead, she gracefully moves from one elegant and challenging position to the next in a seemingly effortless way of going. However, this is only possible because 100 percent of her body's energy is spent working for her. Like a ballerina, a horse that channels their mental and physical energy appears free of tension but is in fact full of a positive tension that separates him from one that is just lying on the ground, sleeping.
For this reason, the second element of the training scale--relaxation--is only achievable when it is built on rhythm with energy and tempo. The key word being energy. That is why a horse that is mentally tense does not always benefit from constantly being slowed and stopped. Often, the proper way to ride a horse that is spooking is to push him past the scary area of the ring. Similarly, a horse that is hot in the beginning of a ride might benefit from going right into a trot circle instead of walking for 20 minutes until he "calms down."
When you think of tension as a positive element of your ride, you then have the ability to realize that mental tension and physical tightness do not have to be stopped. Instead, refocus your horse's energy into a positive and develop it into a quality energy that allows you to increase your horse's relaxation (with elasticity and suppleness) throughout your ride.