Although the term fisting
may sound violent, the act is not. Fisting is just the name for inserting one's whole hand into a woman's vagina. Once inserted the hand will naturally form into a fist shape, hence the name.
Although the vaginal membrane is extremely pliable,(remember a baby's head will fit through) care must be taken in fisting. The woman must be very aroused and lubricated. Always use plenty of lube
For more about fisting, read the Fisting How To Guide.