Hi Pooya,
did you take a look at some of the videos?
To be a little more accurate: Stefan Raab and his team reviewed tons of video material that was taken in the so-called "Casting Box" (http://www.unser-song-fuer-deutschla...ialTab=related).
The candidates sang or played instruments in there to show what they can do.
And when Stefan Raab saw Lena on video for the first time she put a spell on him (that means magic happened )
Okay - in reality she burnt into his memory because of a very fancy song with a special text line at the beginning and her remarkable dancing moves
Search for it!