When sparring, the moment the referee says “fight,” many students stiffen and move around similar to robots. Again, repetition helps; the more you spar, the easier it gets. When attacking, the entire body should be relaxed throughout the movement of the attack, then the entire body tenses for the split second of impact to transfer energy into the target, and then the entire body relaxes again as the attacking limb retracts.One thing I have found that works is to practice on a punching bag. Play some good exercise music and start moving around while concentrating on keeping the entire body loose and relaxed similar to a rag doll. Exaggerate your movements and try to stay super relaxed. Then, periodically, throw a punch or kick, while still trying to remain relaxed. When doing this sort of training, it best to be alone, since you will look silly in your movements.
Breathing is also important in staying relaxed. As you know, once you get winded, you tense and become defensive instead of attacking. To breathe properly, breathe deeply, slowly, and rhythmically, in through the nose, out through the mouth, using the diaphragm. One way to judge your breathing is to watch your abdomen. When breathing correctly, when you inhale, the abdomen should swell outward as the diaphragm drops and draws air into the lungs; the chest will expand atomically to accommodate the extra volume of air. As you exhale, the abdomen should pull inward as the diaphragm rises to expel air from the lungs. When told to take a deep breath, most people suck in their abdomen and expand their chest, the opposite of the way the system works. The intercostals muscles between the ribs were not designed to be the controlling factor in breathing, they merely assist.