(Your mileage will vary. I realize some people can just throw a fist right in and some people could never do it in their lives, and some people are in the middle but the experience is in some way totally different for them. So these are really just things I learn.)
Trust. Someone with their entire hand inside your vagina could hurt you, deliberately or accidentally. You have to trust, not just that they aren't evil, but that they will read and respond to your signals and that they'll put your pleasure and safety above everything else. This is a trust that goes beyond being sure they won't go "now I've got you, mwa ha ha", and into being sure they won't go "but I've almost got it, just let me try a little longer pleeeeeaase."
Planning. You can't spontaneously decide to start fisting, and next thing you know, his hand just happened to be in there. You have to lube and relax and gradually add one finger after another. You have to talk about it. You have to work at it. This is an entirely intentional sex act.
Relaxation. You have to be way relaxed. Like nearly-asleep relaxed. If you're amped up, positively or negatively, your vagina will know. You can contract it at will like you can contract any muscle, but relaxing it has to come from somewhere deeper. Your fingers and toes have to be relaxed. Every worry and stray thought can be a twitch in your vaginal muscles. The right state of relaxation is very nearly a trance.
Communication. This isn't something you can just consent to, go "okay, stick your hand in me" and lie back. This is something that will make you acutely aware that even the best partner isn't psychic. They can do their best to read your face and your muscles, but they can't feel your pleasure or pain or minor discomfort. For them to know, you have to give constant, active, clear verbal feedback. "I'm ready for one more finger." "That hurts a little, give me a second." "Go slow slow slow, but keep going."
Humility. Sometimes it just doesn't work. All your beautiful trust and communication is for naught, because your vagina just doesn't have that extra half centimeter in it today. You have to be able to say "that's okay, I had fun" instead of barging ahead into frustration or pain.
Anatomy. Having a hand in you makes you acutely aware of your pelvis. You can feel the structure of your own vagina with incredible detail and experience its strength and resilience like never before. Fisting outlines in stark detail how your vagina presses against your bladder in front and rectum in back, and exactly where your g-spot is, and your cervix. Even the bones of your pelvic girdle suddenly become a very real consideration. The things that you're vaguely familiar with from diagrams become physical sensations.
Open-mindedness. The experience is nothing like what you think. I wouldn't even necessarily class it as a BDSM activity--the top has to be extremely responsive and ultimately obedient to the bottom, and when all goes well there is no pain. Before I had experience with fisting--even though I'd had my fingers in vaginas--I always thought of it as working with the pain tolerance of a basically stretchy gooey organ. It's not. It's working with how far an organ of rock-hard, unforgiving muscle will let you go. It's not a domination of the vagina; it's a meditation on the vagina.
Awe. There's just nothing like that moment of "holy shit it's in holy shit." Nothing like it on Earth.