Smart Reflections in After Effects
Reflections are great examples for demonstrating the purpose of an old infamous After Effects trick:
By placing an empty adjustment layer between two 3D objects that would normally obscure each other, you can force the renderer to use the layers index in the timeline to determine which layer is rendered in front of the other, instead of it being defined by their actual z-position. That means that even if a layer is spatially located behind another layer, once put on top of an adjustment layer that is between both 3D layers, it will be forced to render in front of the other.
Using this trick for reflections
The way reflections usually work
The standard renderer in After Effects doesn't support real reflections, and even if they were, they wouldn't be very practical in a lot of cases. It would force you to position layers behind the camera, while constantly switching between the camera perspective and something like a top-down perspective to dreadfully navigate both layers step by step to get the desired result. Or the reflection layer might even mess with other foreground elements.
The way we make them work here
We can, however, use adjustment layers as described to create a more "WYSIWYG"-workflow, where everything can be adjusted from the camera perspective. Instead of placing the reflections, where they normally would be, we position them behind the mirror, while preserving the absolute distance to it from before. We add the adjustment layer between both layers. The reflection layer is on top. To limit the reflection to the mirror frame, we then duplicate the mirror layer and use the cop as an alpha track matte for the reflection layer. Voilà!