The variation of the main riff appears at the beginning of the guitar solo in the B section. It is a four-measure phrase, with the main riff used for the first two measures. The second two measures include a transposition of motive x up a perfect fourth to A, and the phrase ends with power chords on A and C.
By following the main riff with its transposition to A minor, the harmonic quality of the original idea is changed. The original two-measure riff is harmonically static, residing only in E minor. With the transposition, the four-measure variation replaces the static harmony with a power-chord progression from E to A and eventually to C at the end of the riff.