http://www.sciencebase.com/science-blog/classic-chords-3-the-hendrix-chord.html

There’s one chord every wannabe rock guitar hero has to figure out at some point…we all listen to Jimi Hendrix, we all marvel at what he’s doing with that Strat, whether plucking it with his teeth or setting it on fire. But, what is it he’s doing exactly to get that E-major power chord that is something like a dominant 7th instead of a no-third, but has a little bit of funky extra hidden behind all that disortion. Well, it’s the pinkie finger on the B-string at the 9th fret that gives it the unique Hendrix flavour.