Preserve Underlying Transparencies

Right next to the [blending] MODE option of any layer is a little box simply labeled “T”. This is the “Preserve Underlying Transparencies” box. Click it to activate, and After Effects will take the alpha channel (or “transparency”) of the layer directly beneath and it apply it to the current layer.

 

For more info

http://sparksight.com/blog/quick-tip-for-motion-graphics-artists/