Apple Shake 4 - Manual (Page 687)

Chapter 24 Keying 687
The following example uses a SwitchMatte node to assign the information from the
combined keys to the foreground image. The resulting combined image data is then
composited against the background using an Over node.
Blue and Green Spill Suppression
Once the composite is pulled from the keyers and put back into the KeyMix or Over
node, you can start to work with spill suppression. Blue spill occurs when light is
reflected off of the bluescreen and onto the foreground material.