In the following riff, both motives undergo variation: the clean-tone timbre of the guitar is switched to distortion, the first motive is filled in by a second attack on the D power chord and a D# power chord (creating chromatic movement), and notes of the second motive become root notes of power chords. The drums and bass enter along with this riff.
Notice how the idea evolves over the course of the variations. The sparse beginning motive leads to an additional motive that adds to the character and complexity of the riff. Both of these motives are developed with the additional notes, power-chord harmonies, and the switch to guitar distortion. The resulting version becomes the theme of the song, appearing in between the separate verses and after the chorus. The first two riffs continue to be used as well; they appear in order during the verses, so the same process of unraveling from the simple motive into the theme is repeated throughout the song.