How does Derren Brown use subliminal messages?

The same way anybody else who has dabbled in sublimes.

Messages are hidden inside TV adverts. These adverts could be innocent-looking text or hidden audio in the background. Your brain unconsciously picks up on these sublimes and processes them.

Many experiments have been preformed with subliminal messages, including by Derren himself, most which has proven that sublimes work.

Ever walked into a shop not expecting to buy anything and then thinking "oh, I feel like getting a pair of shoes. I'll go look now"? That was probably a subliminal message hidden in the audio playing in the shop background.