About compound effects and control layers
Several effects rely on a control layer as
input. These compound effects use the pixel values
of the control layer to determine how to affect the pixels of the
layer that the effect is applied to. For example, the Displacement
Map effect uses the color values of a control layer to determine
how far to shift pixels of the underlying layer, and in which direction.
The compound effect ignores effects, masks, and transformations
of a control layer, unless the layer has been precomposed with these
items. It is common to use a control layer that is not itself visible—that is,
its Video switch 
is off.
Most compound effects include a Stretch Map To Fit option (or
a similarly named option), which temporarily stretches or shrinks
a control layer to the dimensions of the effect layer. This provides
a pixel in the control layer corresponding to each pixel in the
effect layer. If you deselect this option, the calculations for
the compound effect are performed as if the control layer is centered
on the effect layer at its original size.
For many compound effects, neutral gray pixels
in the control layer correspond to null operations. Therefore, a
neutral gray solid layer a good starting point for creating a control
layer. Apply the Fractal Noise effect or Turbulent Noise effect
to a layer and precompose it to create a good control layer for
turbulent or atmospheric results.
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