Being tense and not relaxing is problem for many martial artists; actually, it is a problem in all sports. Ever noticed how you are able to kick and punch for 30 minutes during class exercises and not get winded, but after 3 minutes of sparring you are wasted, even though you used only a few kicks and punches. The controlling factor is whether you are relaxed or tense. The best fighters seem unconcerned while fighting; they actually seem bored. Instead of tensing and concentrating on the opponent, they just relax and let their trained reflexestake over.
You do not think about applying the brakes when a child runs in front of your car, it just happens. You have performed the braking motion so many times that your body has developed “muscle memory” and it responds unconsciously. To be relaxed while performing the martial arts, you have to have performed the motions thousands of times. An old saying in karate is that “to know a pattern, you must perform it a thousand times.” It is not that you gain some great insight from performing it this many times, it just that the movements become as natural as breathing.