We often use a "three fingers" way: index and medius under the ball, thumb over it.
This gives her more power than a two fingers method.
It prevents the ball from slipping off.
It allows to use either the flesh of the thumb or the nail.
It allows to move the thumb, searching the most sensitive position, and then to dig in seriously there.

I must add she has practiced tennis a lot and has a tremendous grip in her right hand...