The front kick=
Just bend your knee and extend it? Right?
Wrong, you're just going to hit air that way.
Well, how do you kick hard and fast then? First off, you have to shoot up with your knee in order to kick that area, you just can't shoot your knee down low, and expect to come high.
You need to use your hips to shoot out that leg - Contract the same hip with the knee kicking, and extend the other to make your other hip shoot out more.
The sidekick/roundhouse.
You're going to do the front kick, but you're going to extend both glutes and use your pecs to turn your whole body in order to gain distance and even more momentum to go up to the body/pec and dm some people.
You're going to turn and bend the chest and lumbar with your leg you're going to kick with (Right turn/bend with right contracting).
As for pecs, it's mirrored on the other side.
(Left pec contract, Right pec extend / Right turn chest and vice versa).
Now that you have those basics down, I'm going to tell you what you can use to change directions and angles of your kicks.
Ankles, hip direction, and glutes can be used in this case to either add a blunt force effect, sharp forceful, or a blunt build up.
You want to use blunt force to push your enemy away for a retreat or to secure your point.
Sharp forceful is for finishing the fight or starting it.
The blunt build up can be used for a combination, follow up kick, or grab - But remember; as you attack, they can attack back, so be aware of counters/grabs coming at you.
Is an example of one opener used to create each of the effects.