When Gatsby arrives to pick Nick up to drive into the city, what mannerism does Nick describe as Gatsby balances himself on the dashboard?

From the text:

He was balancing himself on the dashboard of his car with that resourcefulness of movement that is so peculiarly American--that comes, I suppose, with the absence of lifting work or rigid sitting in youth and, even more, with the formless grace of our nervous, sporadic games. This quality was continually breaking through his punctilious manner in the shape of restlessness. He was never quite still; there was always a tapping foot somewhere or the impatient opening and closing of a hand.