This effect allows you to do Chroma Keying (also known as Green Screen or Blue Screen) in Kdenlive. Chroma keying is where you remove backgrounds of a similar colour. This effect is a very basic. For a more complication alpha manipulation/background removal effect, use Colour Selection
For black backgrounds, the screen transition is slightly more effective.
Here are some video tutorials on how to use chroma key.
Basic Chroma Keying Tutorial
Note: Adding a composite transition is no longer required.
Chroma Key tutorial outlining touchups
Note: This video is outdated. In newer versions of kdenlive have the Key Spill Mop Up effect installed by default, and it is no longer required to use a composite transition.
See also Rotoscoping effect. Rotoscoping is where you manual draw a region, and everything outside/inside that region will disappear. This is useful for backgrounds with multiple different colours.
See also Key Spill Mop Up effect in the Misc group. The keyspillm0pup effect can be used to improve the edges of the blue screen effect - when edge problems are caused by "keyspill". Keyspill is when the color of the screen used for colorkeying spills onto the subject due to light reflection.
See also Colour Selection which also does color based alpha selection, but in a much more detailed fashion. Use it for less contrasting or more complex backgrounds.