Looking at the illustration below, you can see the muscles and tendons around the anus form a figure 8 that then encircles the vagina. At the top of that figure 8 is her clitoris. As you spread her asshole with your cock, those muscles actually pull her vagina closed and stimulate her clitoris. That's right, by fucking her in her ass, you stimulate her clitoris. This is how a woman can obtain an orgasm through anal sex, without vaginal stimulation or direct clitoral stimulation.

As you can see in the illustration below, the g-spot is located where the X is. To stimulate this nerve bundle, it needs to be pressed against the pelvic bone (the white area located on the other side of the urethra and bladder). When a cock enters the vagina, it does little to press against this area. If you notice what looks like a pink wad of chewing gum located to the right and below the X, that is the firm tendon/muscle bundle between the anus and the vagina. When your cock enters her ass, it presses this firm bundle against the g-spot, providing better g-spot stimulation than a cock in her vagina. I also like this illustration because it looks like it's x-ing out the vagina like, "don't put it there."
These are some of the reasons a woman can obtain a stronger orgasm from anal sex then vaginal. Although it involves the clitoris, women I've talked to say it's different than a clitoral orgasm. Where the clitoral orgasm is shallow and focused around the clitoris, the anal orgasm is deeper, more connected, and involves her anus and rectum, her vagina, her g-spot, and her clitoris.

If you think the vagina seems lifeless compared to the rectum, the video below is proof. It's plain to see how it's her anus, not her vagina that plays the biggest part in her orgasm. Notice how her anus pushes out and opens, then tightens and contracts? That creates a natural sucking feeling. It also demonstrates how the anus better stimulates the cock and provides both parties the greatest amount of pleasure. It's as if her empty anus is begging for a cock to fill it and deposit it's load where it belongs.
http://www.xnxx.com/video202474/great_orgasm