Watch the The Relative Minor online guitar lesson by Vicki Genfan from Acoustic Rhythm Survival Guide
Using the relative minor is a great way to bring a new 'feeling' to a melody. Because the notes in the original chord and its relative minor are almost the same, you can keep a melody going while changing the chords underneath it. Listen to the example I use of the Rolling Stones, 'You Can't Always Get What You Want'.
TRY IT with a song you know that has 2 or 3 chords in the verse or chorus. Pick one chord at a time and substitute the minor. It's a nice effect for a verse or chorus later in a song that might help to build the energy.
And remember, you can simply 'substitute' or you can 'add' the relative minor as a way to bring more movement to a progression.