Apple Final Cut Express HD - Manual (Page 790)

790 Part IX Effects
Darken
Darken emphasizes the darkest parts of each overlapping image. Whites in either
image allow the overlapping image to show through completely. Lighter midrange
color values become increasingly translucent in favor of the overlapping image, while
darker midrange color values below that threshold remain solid, retaining more detail.
The order of two clips affected by the Darken composite mode does not matter.
Suggested uses: The Darken composite mode is useful for using one image to
texturize another selectively, based on its darker areas. You can also use Screen for
variations on this effect.