>>82360975You don't just shove your fist in there from the start, anon. Fisting is more of a plunging act than anything else.
To safely fist someone or be fisted, you first need to touch your index finger and your ring finger together with your thumb beneath the middle finger. Then the formed hand slowly slides into the orifice of your choice, and once fully engulfed, begins to form into the fist. Once the fist is safely formed, pull and push your arm gently in order to "plunge" the body to create extreme pressure sensations deep inside.
When you're finished, you form the first gesture again to safely slide out.