Watch the The vi Descending Half Steps online guitar lesson by Ravi from Guitar Lab: 30 Rock & Pop Progressions
This is a minor chord progression where the bass note descends by a half step on each change. The vi chord (some might refer to it as the i chord, minor instead of major) itself doesn't change but by walking the bass note down in half-steps, it creates movement and suggests a vi-V-V/V-IV. V/V is spoken as "five of five" which means that it is the fifth degree of the fifth degree of the key. In the key of C, that would be D major or possibly D7 since the V chord is often a dominant chord. G is the fifth degree of C, and D is the fifth degree of G, hence the V/V. This is an effective way to extend a progression with a chord that is outside of the key (D in the key of C is always minor, but here it is major since it is the V/V). The V/V will usually resolve to the V, and the V will usually resolve to the I bringing us back from our little journey outside of the key! This progression departs from that more common resolution and instead resolves to the IV because the V/V is being used to create a descending line and hint the chord changes vi-V-V/V-IV.
Examples: 25 or 6 to 4, Dream On, While My Guitar Gently Weeps, Your Song