This is the grip to hold the racquet for the serve, volley, sliced backhand and smash.
It is known as the chopper grip, because it is the way you would hold an axe.
It allows you to swing the racquet head faster when serving, which is how to generate power and spin.
To find the grip, hold the racquet as if you were using the edge of the frame as an axe, that should be the correct grip.
The V of your thumb and forefinger should be just to the left of the top of the grip.