Interesting tennis analogy in the third paragraph of this excerpt from a good ezine article on whole body movement (that he calls Body Unity) by a US Marine lieutenant colonel:

Why People Speed Up

Let’s face it: when moving slowly with a training partner, you can easily counter the other person’s movements if you suddenly speed up. If in reality, you were already five to ten times as fast as a normal human being then you wouldn’t need anyone’s fighting system because you’d be Superman. The real challenge is in finding and defending openings while consistently moving slowly–and that is a tremendous source of growth in your spontaneity training.

People speed up mostly due to ego, so no matter how slow you tell them to move, they just cannot resist trying to “win.” This throws off not only their timing, but their body unity as well, since generally when people speed up it is just their arms that go faster. They become disconnected from their root so their strikes lack power.

The other reason people speed up is that they intuitively feel their bodies are out of position to strike effectively. As a result, they try to “catch up” by moving their arms faster, rather than aligning their bodies to a better position. For those who regularly attend our classes they have often heard me discuss that it is not so much a matter of speed but body positioning. It’s just like playing tennis with a good tennis player: they don’t run, they make you run! They play position and cut off angles, make you play beyond your “Sphere of Influence” (a critical self defense concept). When you’re close to the net, they knock it over your head just inside of the back line. When you’re far from the net they “chip” the ball just over the net and make you sprint to return their serve. If you’re to the far left they hit the ball to the near right and so on… The same applies in self defense when you have superior Body Unity.