Hello, I don't post much on these forums but I think I can help here. (: Mannequins are a great idea! Visualizing is very hard.
First a link that might help, the second point is about reflections in the water, though the whole article could be helpful. Click.
Now for my take on it: Simply copy-pasting won't look right, since that's not how reflections work. It's a quick fix that works in certain cases (horizon line in the middle of the picture and mirror and "reality" meet there), or if you're a stressed artist (though who isn't?).
What we see in the reflection is the parts of her that face the mirror at the same angle as we are looking at the mirror at. The perspective in the reflection matches up with the perspective of the real world. That means that you can (should?) draw a mirrored twin of her sitting butt-to-butt with her upside down in that reflection, as you'd see her from below. This is also the reason you should not wear a skirt when walking over a mirror floor, unless you like everyone seeing your panties.
Have a look at this picture. Look at the people in the reflection - they're not distorted in any way, nor a copy-paste of what we see of their "real" selves. Instead the reflection has the same vanishing point downwards as the real person, and the parts of them that we see are those that - as mentioned before - face the mirror at the angle we are looking at the mirror at. That means: we see the soles of the man in the reflection, which we cannot see on the real person. Or look at the curves of the edge of the woman's shorts, on her real legs the curve goes down, indicating that we see them from above. In the reflection, however, the curve shows that we are looking at the shorts from below!
Second, clearer example! Where we hope it's a fake fur. Here you can see how the perspective matches up on the box, for example. The real and mirrored "upper edge" and text all lead to the same vanishing point.
Look at the tiger's mouth: only in the reflection you can see the upper palate.
Or look at the model: You clearly see her from below, and you can see parts of her in the reflection that we don't see on the "original": Her clavicles, the underside of her haircut and bust...
Remember from physics class how light rays bounce off a smooth surface at the same angle as they hit it at? You can imagine a table tennis ball too, if it hits the table from a 30 degree angle it will bounce off at that angle too. Taking examples with mirrors: When you look at your own face in the mirror, you're looking at it from a 90 degree angle, which is why you only see yourself. Turn the mirror around and you'll see what's at that new angle. When you look around a corner with a mirror, you're making use of that fact too: Hold the mirror at ~45 degrees and you can see what's going on behind that corner!
I hope this helps a bit.
