As I said or implied, much depends on 'what happens' with the fingers, hand or 'fist' when he is using this as part of your foreplay. Are there 'dangers' or 'risks'? I would think that, depending on the amount of 'force', pressure, etc he uses, will determine 'potential problems'. At the very least, it will keep the vagina stretched out. COuld this potentially lead to problems of vaginal or uterian prolapse... I would think that is also a potential and very high risk. At the very least, I suggest talking with your GYN as he would be 'the expert' on YOUR BODY since he is lookng, seeng and can give direct feedback about YOUR body. Can SOME women tolerate fisting without problems? I suspect there are. Many or most would find it painful or damaging. LIke most things about sexuality, there are individual differences about risks, acceptable behaviors etc.
I would pay attention to 'how sore you are' the next day as a benchmark for potential risks. PAIN or BLEEDING are other benchmarks.. It is your body and you are responsible for what you do or allow done to it. From what I have read, things like hygeine, a good manicure, etc are considerations. Remember, depending on 'how tight' or the 'SEAL' that is made by your vaginal muscles around his wrist will create a great force or suction on the cervix and uterous. THis is the BIG RISK when women use objects ( bottles, some dildos) that create a 'suction' when being pulled out or extracted if they get lodged or 'stuck'. it is sometimes necessary to drill a hole in the bottom of a bottle to release the suction.. Also, with sufficient insertion force, his fingers, hand or fist will be slamming up against the cervix (opening into the uterous) (NOT GOOD).. At the very least.. USE A GREAT DEAL OF CAUTION.. YOu may want to read the following: http://www.xandria.com/learn/sexuality-questions/Is-fisting-safe%3F.html