A variation of the riff follows, in which only the ascending portion of the melodic motive is played (here on eighth notes). The riff ends with the Bb and F power chord pair from the earlier riffs.
In this series of variations, the melodic elements are gradually removed while the tempo of the riff is increased. The main riff adds a new melodic concept (the A/Bb pairing), and that too is truncated in the following riff.
Though these three examples from “The Thing That Should Not Be,” “Enter Sandman,” and “That Was Just Your Life” articulate the same basic concept of introducing and evolving a riff over the course of the intro, they do so in varying levels of complexity. “The Thing That Should Not Be” follows a linear approach, beginning with the simplest of motives and building up in increments to the main riff. Conversely, the introductory riffs from both “Enter Sandman” and “That Was Just Your Life” are rhythmically and melodically deconstructed before being built up again into the main riff of the song, and these transformations are complicated by rhythmic and melodic elements coming and going without contributing to the evolution of the main riff. The approach in “That Was Just Your Life” involves the added elements of jagged tempo changes and shifts in phase lengths, and the result sounds like the band “jamming” through the intro and experimenting and extemporizing until settling on the main riff. Hence, even though all three examples demonstrate a basic function of introduction and evolution of a theme, they also demonstrate that this function can range from a straightforward trajectory to an unpredictable and exploratory approach.