This trick is often done on the lower strings, which are in many cases in a drop tuning. The looseness of the de-tuned string allows for a very wide vibrato, while the thickness of the lower string produces a rich, full sounding harmonic. This effect can also be done on other strings, but each string will vary in timbre . (lower strings will sound thicker, higher strings will cut through more)

This effect is often enhanced in the studio, by adding a second track that doubles the original guitar.

Effects: heavy distortion