You Should Always Attack High and Low Together
This is a pretty common concept amongst martial artists. Let’s face it, if your being attacked in more then one place at a time, it is harder to defend then just a single attack. Untrained fighters just aren’t aware at all, and are much more vulnerable to this approach. If they see, and defend one, the other attack will most likely succeed.
There is a concept in Kempo known as walking techniques. The idea with it is that as you move/close with your opponent you put your feet in close to your opponents feet/leg on the first move. There will of course be subsequent movements (at least a second move, you disrupt them on the second move). Once you close follow the previous methods I’ve talked about for positioning, skeletal freezing, timing, etc. if you put your foot very close or on top of the opponents foot, it allows you to control them with your leg as they try to move again. Again you can stop them from moving, take them off their feet, or break things, as they try to make their second move.
Kicking the leg as they try to move, or sweeping them as they move, works well if you want to be more aggressive then just stopping their rotation. Of course you’re still doing everything you should with your upper body attack. You need to learn these different principles, ingrain them until they become unconscious things for you. You keep adding in principles this way. It is like layers of an onion.
Of course this isn’t the only method or system. If you have seen Chen Taijiquan practioners do push hands, you will see them with opposite legs forward, working sensitivity with their legs. While doing push hands they are also pushing with their legs, and will take advantage of an opening either high or low, whichever presents itself.
Other approaches can appear to be more aggressive. For example many systems, most Kempo systems included will fight from a further distance, and intend to immediately resolve the conflict, on contact through the fist move, or flow of moves. Though you need to hit which ever target presents itself, as a general rule it is preferable to hit with the hands, then the feet/legs. The reason for this is that if you kick them on your way in they may not be in reach for your hand/arm attack.
One of the key things I think you need to do to use this approach, is to unbalance your opponent on contact. You want to break his balance and set him in place with your hands, exposing the leg nicely stretched out, locked in place and ready to attack.
If you have ever seen Liu Ho Pa Fa being performed with that 30 degree lean they do, it is perfect for this. They come in from an angle, break the opponents balance, set and stretch out the opponent. Then they follow up. I look at it like using you hands similarly to sticking a fork in a piece of steak to hold it in place, while you use your legs, like a knife to cut off the piece you want.
There are lots of variations on this theme, but you need to think three dimensionally, not two dimensionally.