I assume you're talking about how if you raise your right hand, it looks like your reflection is rasing its left hand... This doesn't have anything to do with a horizontal plane or anything... You could tilt your body at any angle you want and it would still happen. What this has to do with is the fact the the mirror image is basically flipping the plane parrallel to it backwards. Nothing is actually flipping to another side, which is why when you put your hand up to the mirror the reflection and your hand are in the same place... It's like if I was where the mirror was and I was looking at you raising your right hand, it'd be on my left side. You're just looking at yourself from the mirror's point of view instead of your own, and the reason that it looks like the reflection is raising it's left hand is because you're seeing the front of the person's body, when actually you should be looking at the back. So, if you think of it as looking at the back of the person, only with the face and everything on it, you'll see that it's really the right hand... Or something. I don't know how much sense that made.