One of Derren Brown's greatest achievements is correctly predicting all the six national lottery numbers. He wrote his predictions on six balls and they all turned out to be correct. However, he hasn't revealed his predictions until after all the numbers have already been announced. The balls are in full view all the time and no one comes near them.
Derren has revealed the method he claims he had used in his next show. It involves a combination of automatic writing and the so called "wisdom of crowds" phenomenon. However, we don't think this is what really happened. You can see his explanation here.
So, if Derren's explanation is false, how did he really do it? There are several options, but one seems to be the most plausible: split screen.
What does this mean? On the left side of the screen (the one with the predictions), what you see is actually a pre-recorded footage. The only thing that is actually transmitted live is the right part of the screen with Derren and the TV screen. That way, we cannot see what is really happening with the balls. We only see a previusly recorded footage of them. Meanwhile, Derren's assistants can put the winning balls into place just in time for Derren to come over to the left side of the screen and reveal them. When the balls are in place, left part of the screen also starts showing live footage.
Both parts of the video are cleverly put together so we think we are looking at one, continous live recording of the event. The camera is even moving all the time so we wouldn't think of this explanation, but the movement effect can be easily achieved.
Here is a video demostration of a split screen for easier understanding: