After many correct predictions, Derren asks the woman to put all her savings on one bet. She does it, trusting that Derren's method of prediction will work again, just as it has worked many times in the past.
However, there is a twist and Derren hasn't exactly told her everything …
Watch the show if you haven't seen it yet, or just to refresh your memory. Then, we will explain what still needs to be explained at the end of the show.
However, since we know it is impossible to predict such things, there must be something else going on. So how did he really do it?
Here is the most probable explanation: he used his own money (probably supplied by whoever finances the show – they can afford such things) and bought tickets for every horse in the race. Then, after he saw which horse won, he used sleight of hand to exchange the losing ticket for the winning ticket. This obviously isn't shown in the footage, making it look like the woman was holding the ticket in her hand all the time.
A less probable theory is that he did the same thing with many people, always placed a bet on the horse which was most likely to win, and only showed the footage of the time when it worked.