It's a oscillating fluctuation in the power delivery in certain situations, usually around 3000rpm under brisk but not flat-out acceleration. When it happens, it only lasts for a few seconds but it's quite annoying. For the me, the most typical situation it happens in is on bend: I slow down on the approach, then re-apply the power to take the bend under gentle acceleration, but unless I re-apply the power very smoothly, I get this slightly lurching acceleration for a few moments.
If you search on these forums for "yo-yo" you'll find loads of hits. A particularly good discussion is here:
http://www.mini2.com/forum/showthrea...ighlight=yo-yo. If you've got nothing better to do and like reading, the "Yo-yo Chonicles" on northamericanmotoring.com makes fascinating reading here:
http://www.northamericanmotoring.com...ad.php?t=20115.
For a long time BMW denied there was a problem, then last year they came out with an alleged fix in the form of a software upgrade (CD38). It cured it for some but not for others. It's pretty clear (especially from the "Yo-yo chronicles") that there's a hardware aspect to the problem too, which they don't seem to have admitted or fixed.
The good news is that if you don't know what it is, you almost certainly haven't got it!
Alan