If you ever watched the Bruce Lee film Way of the Dragon, you will remember the part when he performs a side kick on a person holding a body pad.
When Bruce kicks the pad the person goes flying through the air into a stack of well placed boxes.
To accomplish feats like this, strong legs are a must! and squats are the way to obtain them.
To be able to generate a hard shocking roundhouse or side kick requires strength in the legs.
Training to obtain this strength is very hard without the use of weights and because your legs are big and powerful you may require a lot of weight, which means going to the gym.
Another alternative is the Squat on one leg.
There easy to do, and can be done anywhere.
I like incorporating them straight after practicing kicks on the heavy bag.
Sometimes you see some people doing these by just extending there leg, as if holding a front kick, and squatting on there supporting leg in that position.
I don't recommend this way as i find it places a lot of stress on the squatting knee.
Firstly i use a wall for balance (you don't have to use a wall but i like to work strength and balance separately).
Then i get into the starting position which is the chambered position of a roundhouse kick (this will turn your supporting foot slightly).
Holding the wall with one hand and the chambered foot with the other i start the squat, going just past parallel to the floor with my thigh.
To finish the movement, i push so my leg is straight again (not locked) and perform a roundhouse kick with the chambered leg finishing again in the chambered position ready to squat again.
This should all be done in a slow but fluid motion.
Usually i do this for ten repetitions each leg, and 4 sets.
It's a real burn in the thighs but excellent for building raw strength.