The partly agree with Cheetah in that it is about "lift" with the arm, at least for most players.
BUT, in order to achieve that "lift" you need to hit further out front so that your arm is in the upward arc of of the swing. If you hit close to your body, at the bottom of the arc, the racket has no lift.
Therefore, you need a grip that allows you to hit sufficiently out in front. Rolling back your heel pad is the easiest way to achieve this.
As I've said before, it makes no sense to only use the index knuckle as a measure of grip. There's a huge difference between an Eastern with the heelpad on top vs. heel pad on the back bevel. Anyone who tries this can feel it.
Since I've stated all this recently, here's a link