Often, newbies to Boxing or Kickboxing will have their chins up when they spar.
You need to keep your chin down.
Keeping your chin down protects your nose.
If you get hit, you want to minimize the amount of damage you take on.
By keeping your chin down, your nose will be further protected as the bridge of your nose will be vertical to the ground. When you keep your chin up, the point of your nose sticks straight out and the bridge of your nose is on a slant. Because the profile of your nose is so pronounced at this angle, your nose will surely break if you get hit straight on.
As well, your chin is further protected if you keep it down.
The reason you want to protect your chin is that your chin acts like a lever and when it gets hit, it turns your head like a spinning top.
Your brain sloshes around inside your skull and smacks against one side, bounces off, and smacks against the other side. This could lead to a concussion, which is basically a bruise on the brain.
By tucking your chin in as you fight, if you get hit, the lever is then much shorter. Your chin is closer to the pivot point, your spine. And thus, it is much more difficult to turn your head via your chin if your chin is tucked.
You can experiment with this.
VERY CARFULLY, push and pull on your chin and on the top of your head with the chin down and then with the chin up.
Once again, DO THIS VERY CAREFULLY.
You’ll find that your neck is much stronger and much more stable with your chin down.
Also, by keeping your chin down, you are enabling yourself to look at your opponent’s chest. This is critical for kickboxing so that you can see both the feet and the hands of your opponent.
Furthermore, by keeping your chin down, you’ll be able to move your head easier. When you are moving your head as you evade a punch or a kick, you need to move your entire trunk not just your head like you are Will Ferrel from Night at the Roxbury.
By keeping your chin tucked in, you’ll be able to move your head easier because it is tucked in around your shoulders.
Look at Mike Tyson. He looks like he doesn’t even have a head.