Keeping balance performing spin kicks
Spinning means you have to take your eye off the target for a period of time. You really need to get your eyes back on the target as fast as possible, this means a quick turn of the neck. Never kick, especially at kup grade level, without looking at the target, this is how accidents happen at training. Also by turning the head and getting your eyes on the target quickly means your back is more likely to be straight.
Keeping your balance also means you need to keep your back as straight as possible. Students tend to throw their bodies back to get their leg higher, to combat this stretch more and try and keep that back straight.