Felt Tips has a quick look at getting more familiar with the After Effects Matte Choker, and covers what the options for the effect are. There is also a quick behind the scenes type of look at what the Matte Choker is actually doing in After Effects to create its effect.
This 8 minute quick tip gives an insight into how the matte choker effect works, and how to animate it smoothly between values and back to a neutral valueFelt Tips
The Matte Choker effect is typically used for compositing, mostly when keying green screen, but as Felt Tips notes, it can also be used for various motion graphics implementation as well. A simple example is shown by animating the matte choker effect on some type in After Effects.