It's a technique for making a beat seem to be always accelerating or decelerating. It's actually quite simple (for what I know): you make an accelerating beat fading in and out, and repeat that same piece making sure one begins when the other is on its maximum. That way the brain "slides" from one beat to another, without taking notice of the change, and thinks its all part of the same beat, an always accelerating one.
I'm not very good at explaining it, but you can found a better explanation onhttp://en.wikipedia.org/wiki/Shepard_tone
And some examples and more info onhttp://gethighnow.com/risset-rhythm/http://swiki.hfbk-hamburg.de:8888/MusicTechnology/826
(I hope there is no problem with linking this resources, if there is, just tell me).
The Shepard tone is actually the same, just "faster" enough to be on the frequency level, and not the beat one. This makes it seem to be an "always accelerating frequency", which is a tone getting higher pitch continuously. But the principle is the same.
When speaking of binaural beats, it's quite funny, as for us the two effects are exactly the same: our 2 Hz sounds are the same as a 2 Hz beat, and the effect is the same if we speak of rhythm or pitch.
BTW, sorry for my English ^^u