In Martial Arts, one of the keys to becoming great is to truly understand how the body is all connected. When you're delivering a punch, for instance, it's not just your hand that's delivering the punch. It's not even just with your arm involved. A proper punch involves your whole body.
Understanding the mechanics of your body really help you deliver your optimal Martial Arts. You should be able to break down even the simplest of moves, like a punch, and identify how each part of your body helps to contribute to the strength of the your actions.
If you've never tried before, give it a try. Pay attention to your body and try to deliver a punch using just your fist and arm. Are you able to stop the rest of your body completely from moving? Do you feel your body straining against you as it tries to move? Do you feel any power behind that punch?
Now add in your torso. Allow your torso to twist as you deliver the punch. Does the punch feel stronger? Do you feel less strain on your elbows and arms and more power and reach with your punch? Do you feel your hips trying to get into the action?
Let your hips go. Do you feel the power even greater now? You've almost engaged your whole body now. You should also start to feel that it no longer is your fist that is leading but instead your body that leads the punch.
One last step, engage the part that binds you to the earth, your feet. This final step helps you to draw the energy from the earth itself into your punch. If you study the internal arts you'd understand that energy comes from the earth and travels through your body into your fist. If you learn to direct that energy, you can send it through your fist and increase your power ten-fold.
In every move you perform in Martial Arts, your body should operate as a single entity, not an isolated segment. Learning to develop that natural movement that allows your whole body to be engaged is key to becoming a master Martial Artist. From an observer and judge's point of view, it's very obvious to see when someone has got this concept or if they are simply moving a portion of their body.