How did Derren Brown (a hypnotist) do the "how to influence dreams" act?

In this episode: https://www.youtube.com/watch?v=sMgTIAUt... Derren Brown used multiple methods to induce a dream. The method I wanted to understand was when he puts his subject into a trance and then plays the video of flashing images with a song behind it. When the subject is sent off to bed and he enters REM sleep Derren Brown plays the music that was played during the video the subject watched when he was in a trance. I am a self taught hypnotist and was wondering why this worked. Did the song become like an anchor (a method used to associate a suggestion with a sound, object ext.) or something entirely different. is it possible to create a similar video put a friend in trance and induce a different dream? any ideas how to recreate this act would be extremely helpful. thank you for your help!