Explain what a 'hemiola' is

A hemiola is a rhymic device that moves music that was in triple time, into dual time. ( clap aloud counting 123123121212). So the chord changes are now happening every two beats, instead of every 3 beats. A great musical example of this can be found in "America" from West Side Story. "I like to be in"- triple time, "America"- dual time, and this device continues throughout the chorus of the song.It gives the listener or audience the impression that the music is speeding up.