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."

 -Frank Underwood's answer when Raymond Tusk asked him that question.