Watching the 2013 Australia Vs England Test series ("The Ashes"), I have seen for the first time an interesting piece of technology which shows the rotation speed of the ball. This is used when spinners are bowling, to give viewers an idea how much each ball is spinning (so you can see if it was an arm ball, stock ball, big turner etc). This is called the 'rev counter'.
Does anyone know, preferably with a reference, what technology this is based on? I would presume some kind of microwave doppler radar, but it seems like a very tricky thing to implement. I don't think it is a high speed camera as the results are obtained almost instantly once the delivery is bowled (as per the speed camera results, which do use a microwave doppler radar).