when we have tension in a part of our body that is not actually necessary for anything that we are doing right now, it means that the tension is there either in anticipation of something or because it has remained from something that the body did previously. in general, parts of our body that can tense up are muscles controlled by our neural network, which means that our brain is responsible for any movement in our body, voluntary or involuntary. so when in yoga you are told to be in the moment - this is what is meant. the body shows us that the past and future are places in the mind that have no real information - that when you apply, quite literally, your force of life and create tension towards past or future, it creates nothing, there is no effect and it is pointless. an effect of experience can only be achieved and observed when we apply our energy to the current moment. the same mechanic also applies to tensions in us that are not so physical, like our attention.