No word of a lie, the first time I did it with my partner was by accident. I thought I was just entering her from behind, when in fact I was actually entering her behind. She didn't say anything because she was really turned on, and this turned her on even more. It did occur to me that she felt particularly tight, and I kind of suspected I might be slightly off target, but we took it slow and easy and it was fantastic. However, when we've subsequently tried it intentionally, it's been slightly uncomfortable for both of us. We find it's one of those experiences that just has to happen naturally, rather than forcing it (pun intended).