The moment that Leon's duet with Kylie had finished was the moment that he won the X Factor because it was the seminal performance of the night. It doesn't matter whether the singing was great or not. It was a moment that had... well it had this little thing called the 'X Factor'. It was the song that anyone wanting to vote would be remembering. In fact, at the very moment that the song ended, I rushed over to my laptop and put as much money as I could on Leon who was trading at 13/1 as the rank outsider because I knew at that moment that he'd won.
Simon Cowell often makes the point to contestants on these shows that even if they have sung well if the song is forgettable it makes them vulnerable. Add on top that if you're the favourite then people will not vote for you because they think you're safe anyway and you can find yourself in a problem as Rhydian did last night. Rhyidan's first song was forgettable. In his duet he was out-powered by Katherine Jenkins, and whilst his last performance was brilliant the damage had already been done.
If it helps think about it like politics and elections. First of all you have the floating voter that reacts to the campaign. Second you you have the problem of not getting your own vote out because they don't think it's necessary to vote in order for the party they support to win Rhydian lost the campaign because he didn't have the "Kylie moment" and his vote didn't turn out because, frankly, they all knew he even if he didn't win he was sorted anyway.
Basically, "it was Kylie wot won it!" and watching it back for a second time just now makes me even more sure of that. The performance was brilliant, natural and didn't feel like something from a talent show final. It could have easily been a Royal Variety performance it was that well done.