Apple Motion 5.1.1 - Manual (Page 232)

Chapter 7 Basic compositing 232
Use blend modes with groups
Blend modes work differently depending on whether they’re used with groups or layers. In
particular, the Pass Through blend mode is available only for groups.
Pass Through
When a group is set to Pass Through, each layer is blended with all layers and groups that appear
underneath it in the Layers list. In this example, the Swirls layer is set to Stencil Luma, and the
Fishes layer is set to Add.
With the enclosing group set to Pass Through, the Swirls layer stencils all other layers underneath
it, including the Gradient layer in the bottom group. The result is that all layers are stenciled
against the background color. The Fishes layer is likewise added to the combined stack of layers.
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