I tried to think of a good explanation. But i'll let this guys analogy try and explain it to you
"He wants to be a planet rotating around you. Imagine that for a moment - you are at the center, and he's revolving around you. If he's at 12:00 moving counter clockwise, where's his motion? To the left. Step to your left and in. You'll find the distance closes very fast, and he has to scramble. Always move in the same direction and in."
So in other words, its all about stepping to where hes going to be, instead of where he is. Get it? That way, he runs into you. He circles left, you take a step right and block off his escape route. He tries to go circle away right, you take a bigger step left to cut him off, your foot outside his. He tries to go move back, you move forward. etc etc. Quite a simple concept.
Once you understand the concept and apply it in the ring, you'll be keeping fools cornered in no time.