This is another concept that I think has been mentioned on here before, and I came across this illustration and thought it was pretty good at explaining the basic principle. There are some other good sites that explain in more detail too that I will post here at another time, but for anyone interested this gives you a good idea.
I use this with mags quite a bit out on walks with rabbits and squirrels and its had an effect as I can easily call her off them now as she looks at me before she legs it, so I can get her to stop before she even starts running if needed. Its quite a handy little concept! :)