How To Kick Hard
Fifty percent of karate is punching and the other half relies on your kicking. And if you are not kicking very hard you are going to be in some deep trouble. In order to kick hard you need to have a proper fight stance. Once you are in your proper fight stance, the secret to kicking hard is the use of your hips. When you kick, you need to facilitate the use of your hips. So when you make contact with the bag, your hips should move along with you. When you are focusing to hit on a bag or an opponent you should imagine your kick going through the opponent instead of just making simple contact.
This video may help you
Sourse: japaneseweapons.org