Being a holder for field goals and extra points is the most thankless task in football.
When's the last time you heard a coach or player praise the holder for doing his job? Does anybody ever call in on sports talk radio to compliment how solid the holder has been all season? When a team wins a team with a kick, does anybody mob the dude who allowed the kick to happen in the first place?
Nobody wants the fate of Tony Romo, who failed to handle a snap in a playoff game and went down in infamy because of it. Eh, things worked out pretty well for ol' Tony, but you get what I mean. Being a holder comes with none of the glory and all of the risk.
So what do you do to ensure you don't Romo it when the lights go up? If you're the coaching staff for the Oakland Raiders, you order a large shipment of tennis balls and go to work.
King was working with the tennis balls during punt drills as well. Does this actually make any sense? Wouldn't it be better just to keep practicing with the same type of ball you'd use in a real-time situation? Who knows -- but I'll grant it's a cool thing to do at a football practice in June.