He found that because of minute differences between the machines, the two phrases moved gradually out of phase with each other. He began to control the discrepancy by delaying one of the spools with his thumb.
This ultimately resulted in the tape pieces Come Out, It's Gonna Rain, and the lesser known Melodica.
John Adams, in liner notes to the Steve Reich 1965-1995 boxed set of CDs on Nonesuch, recognized the contribution of Steve Reich:
... For him, pulsation and tonality were not just cultural artifacts. They were the lifeblood of the musical experience, natural laws. It was his triumph to find a way to embrace these fundamental principles and still create a music that felt genuine and new. He didn't reinvent the wheel so much as he showed us a new way to ride.
Come Out may not be the best work to illustrate that Adams quote. The piece is so minimal, basically a repeat of the phrase "come out to show them" with increasing phasing, such that by the end, it is bizarrely unintelligible albeit still musical. Daniel Hamm's voice provides the base material. After being arrested in Harlem, Hamm needed to prove he was injured in order to get medical treatment. Reich tape looped the recording of Hamm describing the experience, although Reich mostly focused on the "come out" phrase.
Steve Reich on the work (from an interview by Jason Gross):
...This one phrase seemed emblematic. The speech-melody is everything. It then generates all kinds of variations upon itself melodically and on the meaning of the words.
In 1966: Steve Reich had returned to New York from San Francisco; the Metropolitan Opera House opened in Lincoln Center; John Lennon said that the Beatles are more popular than Jesus.