Why does Frank Underwood in the series House of Cards always knock a desk twice in quick succession with his hand right before he leaves a conversation?
"Something my father taught me. It's meant to harden
your knuckles so you don't break them if you get into a fight. It also has the
added benefit of knocking on wood. My father believed that success is a mixture
of preparation and luck. Tapping the table kills both birds with one
stone."