The layer of tissues between the anterior anal channel and the posterior vaginal wall is very thin. Therefore if you push your finger/dick/dildo from the anus in the direction of the G-spot (located +- 2” inside, at the anterior wall of the vagina) you will indirectly stimulate the g-spot.
Women with a sensitive g-spot who enjoy anal play will orgasm like hell because anal stimulation is usually more intense than vaginal. Combine that with g-spot stimulation (and oral clit teasing if you want to drive her totally nuts).
Later - ttt