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Why use the hook grip? It allows you to have faster turn-over in the clean or snatch. In other works, you will get the elbows up faster in the clean; you will get the bar onto a secure position on your shoulders or overhead faster. Read more here at Catalyst and here at IronMind. I can verify what he says about the hook grip both working great and hurting like hell when used to deadlift.
** See demo from Luke http://www.youtube.com/watch?