Phil Mickelson and his caddy Jim "Bones" Mackay have a relationship dating back decades. Mickelson trusts Mackay, and Mackay is like the world's greatest Secret Service agent.
Of course, part of a caddy's job is to know when to keep one's mouth shut. (It's part of the caddy credo: "show up, keep up, shut up.") Observe Bones' reaction when Phil even considers using a driver in the treacherous conditions of Hoylake:
Well done. For the record, Mickelson didn't use the driver. And Bones' Jedi-mind-trick game remains unmatched.