The guy said he could just see images in his mind, those images mean a certain number to him...and then he just lets the answer fly out without thinking about it.
When the scientist dudes where making sure he was legit they one day gave him some colored clay. Told him a number, and then told him to make the same image his mind was giving him with the clay. The next day (without warning) they told him to do it again with the same number...and they where spot on.
The dude had a stroke or something like that as a kid. Since the tv show I've been trying to re-inact his stroke to know luck.