If you go online you'll find dozens of videos of Hubert Sumlin explaining how it's done. Here's one of them.
Sumlin was, of course, the guitarist for Howlin' Wolf. Tonight Mr. Sumlin is bringing his blues to B. B. King's in NYC. So if you're not down in Brazil, enjoying Mr. Berry, maybe you'd do well to head to B.B.'s!
Here he is with another legend, Buddy Guy.
What a wonderful guy.