All the diverse descriptions of fisting in this thread are more or less accurate...
Agreed.
And how do you start? I mean, I'm interested in learning more but I have no clue how to get started. Who to ask to play with my ass... etc...
In my own case, I had read about it and didn't believe it was possible, so to find out I tried it on myself. Although I didn't succeed the first time (I couldn't get past the knuckles), I got far enough (4 fingers), that I realized that it probably was possible, and I became somewhat determined to accomplish it.
In terms of the actual "how" of it, I started by inserting one finger and gently massaged my anus, then inserted two fingers when it felt comfortable to do so, and worked up to all 4 fingers. I didn't have any problems until I got to the knuckles. I just couldn't get passed them. I had the good sense not to force the issue, so to speak, but some time later (a few weeks?) I got it in my head to try again. Again, I couldn't get passed the knuckles, but I enjoyed the ass play enough that I continued to do it from time to time, as a semi-regular part of my masturbation routine.
After...oh...several months of this, one day I just sensed that I was ready to go "all the way," so I made a fist and applied firm, steady, yet gentle pressure, and to my pleasure and amazement, my sphincter opened enough, and I was able to insert my fist up to my wrist.
As others have said, the sensation is very similar to what you get from more, um, "conventional" forms of ass play, but
much more
intense.
Ever since then (going on about 20 years now), self-fisting remains a semi-regular part of my masturbation routine. I would also add that it really helps me relax and is a great stress reliever. I assume that there's some sort of endorphin effect going on.