How many women get to climax from anal stimulation

My BF tried something new on me the other day. We were at the back seat of his car. One thing led to another and when I got on top of him he grabbed my butt. He normally does this, but this time he placed his finger there...and it surprisingly helped me reach climax much much faster. It's now become a regular thing and I just wanted to find out if other women have tried it and why does it assist in climaxing?