Relaxing in Jiu-Jitsu while training is essential to your progress. If you train without the ability to relax, sooner or later you will become exhausted or overtrained. More likely than not, you may also be injured. Training with a super-tense competitive mindset can slow your progress. Your ability to learn and perform on the mat is linked to your state of mind. Which is also linked to your breath. If your breathing is smooth and even, it more likely than not that your movements will be the same. In conclusion, relaxing in Jiu-Jitsu is key.