wheelbase
I think the reasoning behind the assertion is that you are adjusting the wheelbase (of the back end at least). Shorter = more responsive, longer = more stable.
However, unless you have extraordinarily long dropouts, the difference is pretty minimal. Give it a go and see if you can feel the difference; I'm not convinced I've ever been able to tell (though I always used to set them as far forward as I could for appearances sake, and perhaps in the belief it would give me more vim).
Nick