I was thinking the same thing - how, logistically, is that even possible. And really, a mare stood around and just let some guy get underneath her? I'm just not visualising this one (thankfully!).
He didn't have to get underneath her. "Slot A" was conveniently located at the level of "Tab B" for a guy standing up, more or less. He might have to twist his head sideways and work around the very phallic tail.
I did as a vet school prof once if mares have the big O. He didn't know. Are they innervated correctly for that? That seems like a straight-forward anatomy question.
He said it was a girl question. I said it was a guy thing not to ask... for several hundred years of equine anatomy.