The two-minute video clip above is taken from a much longer video, in which Jim Harbaugh imparts his wisdom of the quarterback position on an assembly of quarterback coaches. The entire video is over an hour long, but we've cut it down to the most entertaining portion for the sake of your amusement.
Right on the center's asshole. That's where Jim Harbaugh says the quarterback should put the knuckle of his pointer finger before the snap. He says this repeatedly, as if there isn't a more common thing a person could say. I like cats, but I'm really more of a dog person. A good quarterback should put the knuckle of his index finger right on the center's asshole.