Also, combined with a two-handed backhand, it is almost impossible for the opponent to see which side (backhand or forehand) is hitting the ball. The sides often are equally accurate, and no grip change is required.

What grip do you use for backhand?

The eastern grip is the most commonly used for two handed backhands. For the semi westerns grip, your index finger knuckle should be located somewhere between bevel 6 and 7 (find what's comfortable for you).

Does Federer change grip for backhand?

Roger uses the standard backhand grip which is basically an eastern backhand grip. He switches it slightly for when he slices the ball. This grip allows Federer to hit his backhand with both power and top spin.