Fisting requires an intimate knowledge of the relevant anatomy, as there are many opportunities for injury that can be easily avoided if you know your way around the relevant body parts. Making even small adjustments to your movements can make a world of a difference.
First of all, let's discuss the hand. The most important thing to notice about the hand is where it is largest/widest, right at the point where the thumb joins the rest of the hand. When fisting, you need to keep in mind that the vagina will be most stretched when this part of the hand is moving in or out past the vaginal opening. This is especially important as you are warming her up.
The Silent Duck
The best starting hand position is the silent duck. You make a similar shape with your hand as you would when making a duck-like shadow puppet. This position minimizes the widest part of your hand, making it easier to insert your hand past the vaginal opening (http://upload.wikimedia.org/wikipedia/commons/a/a4/Vaginal_opening_-_english_description.jpg). Then, as your hand enters the vagina, curl your finger tips until they make a lightly clenched fist. When pulling out your hand, uncurl your fingers in the same way, returning the hand to the shape of the duck just before it breaches the vaginal opening.
While it is called fisting, there are disadvantages to actually making a fist when either entering or exiting. This is doubly important if you have longer nails. If you do want to use your fist, make sure the nails are dug into the palm, to avoid scr****g or scratching. Also, be aware of your thumb and how exposed your thumbnail is at any given time. Personally, I tend to rely on the duck as my primary hand position, using the fist sparingly when I want a little extra stretch.
Inside the Vagina
There are a few key areas to pay attention to when fisting. As mentioned above, the vaginal opening is the narrowest point of the vagina, and this will be the place where you will meet the most resistance. If the person being fisted is nervous or anticipating pain, you will need to relax them first, as nervousness will cause the vaginal opening to constrict. Foreplay is essential, including lots of vaginal stimulation.
Once your hand is inside of her, there are a few points of contact that you should be aware of (http://upload.wikimedia.org/wikipedia/commons/7/70/Gray1166.png). The first area to be careful of is the pubic bone (marked symphysis in the diagram). This bone is close to the vaginal wall, making it easy to pinch the flesh of the vaginal wall between it and the bones of your hand. If you are having difficulty, try keeping your arm low and exert a downwards pressure away from the bone with your hand. Also, for some women pressure against the rear vaginal wall is highly stimulating. If this is the case for her, using the palm of your hand to circularly rub the rear vaginal wall in a steady, slow motion can produce a wonderful reaction.
Just past the pubic bone is the bladder. This can feel like a round, harder surface and should be avoided when fisting. Also inside and towards the top is the cervix. Again, try and avoid hitting it directly too hard, as it is possible to bruise the cervix. Keeping your hand positionally lower helps, although as long as she feels comfortable, feel free to experiment.
The Fine Print
Neither of us is medically trained, so please take our advice with that in mind. If you do have any medical concerns, please consult a physician. Happy fisting! :-)
P.S. If you are a medical or sexual health professional that is familiar with fisting, we would be interested in hearing from you.
The Credits for this text goes to : FistingFan