Watch Amy Winehouse, Mark Ronson Create 'Back to Black' in 'Amy' Clip

"When you write a song, you have to remember how you felt," singer says in clip. "You might have to remember what his neck smelled like. You have to remember all of it"

The critically acclaimed Amy Winehouse documentary Amy opens nationwide this Friday, but in a new clip, fans can see the singer belt out her hit "Back to Black" in a private corner of a studio without musical accompaniment. Recorded in the studio during a March 2006 recording session with producer Mark Ronson in New York, it's one of the film's most captivating moments, capturing Winehouse's raw, remarkable talents.

"It was just one of those serendipitous things. I just caught her at that magic moment, and she was just ready to get it going," Ronson says in Amy of the Back to Black title track. "That's why I couldn't understand what everyone else was saying about this procrastinating, troubled artist."

"I had these feelings, these words floating around in me," Winehouse says in the clip. "When you write a song, you have to remember how you felt. You might have to remember what the weather was like. You might have to remember what his neck smelled like. You have to remember all of it."

The film is almost entirely made up of archival footage and personal home movies, and Amy director Asif Kapadia recently told Rolling Stone that he came upon the breathtaking "Back to Black" video by luck. "That came to us purely by chance," says Kapadia. "We heard a rumor that someone was filming during the session, and we eventually found it."

While some of those closest to Winehouse have spoken out against the documentary, Ronson recently applauded Amy. "The really respectful thing about the movie is you are reminded why she was famous in the first place – she was a genius, that's the stuff even I can forget," Ronson said. "I forget that when I played her the piano chords to 'Back To Black,' she wrote the lyrics in an hour. I was blown away; people just don't write lyrics like that any more."