What is the difference between the way a side kick is performed while sparring and when using it while performing a form/pattern?
A side kick, or any other kick for that matter, is performed in the same manner, whether using it in either sparring or in a form. The knee of the kicking leg is chambered high and toward the opposite side until the shin is parallel to the floor. The kicking foot is shaped for the kick; the toes are curled backward and the ball of the foot is pointed toward the floor with the heel pushed outward toward the target. The knee is thrust toward the target with the rear, outer edge of the heel making contact with the target. As the heel makes contact with the target, the hip of the kicking leg rotates over and downward and the supporting foot snaps around so its heel points toward the target. These two snapping actions, along with the thrusting of the knee, adds tremendous force to the kicking motion, making the side thrust (piercing) kick the most powerful kick in Taekwondo. After the kick reaches full extension, the knee of the kicking leg is quickly retracted into the chambered position so it may either be used for another kick or dropped back to the floor.
The only difference between using the side kick in sparring or in a form is that, when sparring, the kick is instantly and quickly, snapped back into the chambered position so it may not be grabbed by the opponent or become snagged on the opponent, so stability is regained, and so another kick may quickly be executed. When using the side kick in a form, the kick is held in its fully extended position for a moment, as if you were holding it for second so a friend may take a photograph. This adds to the artistic expression of the form.
Some instructors teach that when using the side kick in sparring, the knee of the kicking leg should not be chambered high and backward. They teach that the knee should be only lifted straight up so that the kick may be performed more quickly. While this motion may be quicker, it is only quicker by milliseconds, which is insignificant since opponents cannot react or move this quickly. With a full chamber, the kick is not only more powerful when it makes contact with the target, it is more powerful in its motion, which makes it more difficult to defect or stop.
Some instructors teach that, when using the side kick in sparring, the knee of the kicking leg should not be retracted into the chambered position. They teach that the kicking foot should only be dropped back to the floor so that another technique may quickly follow. However, dropping the foot means you are committed to stepping into that position. If your kick failed, the opponent may attack with a counterattack leaving you in a bad position. With the re-chamber, you have regained your stability so that you may step down anywhere you choose, execute another side kick or any other kick, use the cambered leg to push the opponent away, use the chambered leg in a block, or merely drop it back to the floor. At the moment a kick makes contact, you body is extended, off balance, and vulnerable. The quicker you retract the kick, the quicker you regain stability and the ability to defend yourself or continue attacking.