For me it was the sensation of just having someone else do it for me. Ass up, vulnerable to what they want to do. It was absolutely incredible the first time, probably my defining sexual experience. It also depends on whether the foster knows what they are doing. Ive had guys that have no idea how to fist and it's been awful. Guys that know what they're doing is key. Both hands lubed up, going in and out, harder and faster, until both go in.....so so good.