You got it correct. The top layer with keylight will be your alpha matte. Only where you keyed it, will the bottom layer show through, it cuts away the bottom layers green screen. Using this method will remove the noise that occurs when you use keylight.
When you have set up, you might want to select both layers and pre-compose them(shift+cntl+c). That will put the layers into there own comp and in your final comp you can now work on both as a single layer.
Johnny Cuevas, Editor
Thinkck.com
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