That's a rhetoric technique. He can slow down and focus whenever he feels like it. The stage antics are great for keeping attention. When he slows down to make a point, it really grabs the viewer's focus.

It's the public speaking equivalent of using headlines and layout and typography to create a good reading experience.

Teachers (and comedians) do stuff like this all the time. His presenting style is pretty well developed, especially for being so loose :)

Maybe he as Aspergers syndrome like the character Jerry on the television show Boston Legal.