When I was doing kyokushin.
All of my teachers taught me that you must punch using feet-waist.
Boxerdo that same. Thats why they look like dancing.
And one more thing,Kyokushin talk about this a lot but boxing dont.
The fist. Your fist should be formed complete. So you must push up with your fist.
If you fight with grove, its ok. But on the street, your enemy wont wait until you wear a pair of groves.
So, your fist should be complete. And try to hit the sand back with groves basicaly, but you must hit it with your bare fist to train your fist sometime.
Oh,,when you punch jab and straight,always trying to stay your elbow closed for the guard. Many people do that with their elbow opened, so they receive those punchs of their opponent easily.