Here's an insightful comment from that dailykos diary about Obama making that brush the dirt off your shoulders gesture:

"He's signifyin'.

I think this is what resonates with young voters, because that ability to make wry, indirect, playful references to popular culture is something that many in the last couple of generations have grown up with, and see as a regular part of their culture.

Signifyin', too, is different from "dog whistle" politics: if Bush "sends a signal" by dropping references to "Dred Scott" in his speeches, or "wonder-working power," these references are invariably clumsily telegraphed, delivered in dead earnest -- a giant, ham-fisted wink to evangelical listeners who are supposed to respond to these references as if they've been activated or stood to attention by a code word.

Signifyin', on the other hand, is playful, wry, humorous, is meant to link the listener to a much more open-ended range of cultural references. Because "brush that dirt off your shoulder" isn't the same as dog whistle, which links to a single isolated idea (Rapture!) from a single isolated book (Bible!), or a single message of bigotry (special interests!); "brush that dirt" opens on to a whole terrain of hip hop culture, along with that culture's longer, open-ended links to other traditions in infinite directions.

This is the brilliance of Obama. He's not only testifyin', he's signifyin'."

-Dale on Fri Apr 18, 2008

http://www.dailykos.com/story/2008/4/18/143821/284/886/498463

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Click here for the wikipedia article that provides some information about the art & skill of "signifyin."