Preserve underlying transparency during compositing

The Preserve Underlying Transparency option causes a layer to get its transparency from the transparency of the composite of the layers below it in the layer stacking order. In other words, the opaque areas of the layer with this option selected appear only when positioned over opaque areas in underlying layers. This behavior is similar to the behavior of a track matte, except that a track matte can only be a single layer and a track matte must be above the layer in the layer stacking order. (See Track mattes and traveling mattes.)

This option is useful for creating results such as glints or light reflecting off a polished surface.

Aharon Rabinowitz provides a short video tutorial about the Preserve Underlying Transparency option on the Creative COW website: www.adobe.com/go/learn_ae_aharonpreservetransparency.