This is something that Pat Symonds wrote about in F1 Racing over a month ago. While he was at Renault they worked on it but could not get it to work reliably. He reckons that Red Bull already have some such system on the RB6 only the engine not change is not as pronounced as on the RB6. There were quite a few in pitlane that noticed the RBR possibility quite a few months ago - hence this is Mclaren following the RBR lead. I reckon it is a form of traction control but there is a loophole because TC is banned as a controlling of the rear wheels directly - not throttle-ignition.