Render After Effects Shadows on Transparent Layers

I stole these instructions from a Creative Cow Forum post. 

Start with
Some comp with a 3d character or something that you want to cast shadows.
New White Solid. 
Make it 3d. 
Make a light. 
On your character set the material options to Cast Shadows and not to accept light or shadows. 
Set the white solid to accept shadows, not lights and not cast shadows. 
Position in the scene as a floor and Pre-Comp as Floor.
Collapse Transforms
Inside Floor comp set mode to Silhoutte Luma
Back in the main comp make sure you have some background behind the floor. (Black Solid will do it)

The Siloutte Luma will cut into the Black Solid giving you transparency with just a shadow. If you need other parts of your scene rendered just go back into the floor clip and create more white solids to take up space.

Your layers should look like this
Main Comp
Light
Character
Floor Precomp
Black Solid

Floor Comp
White Solid for Floor (blend mode set to silhoutte luma)
Other Solid for wall (if desired)
...