This is a great look at using expressions on a single anchor point in a Mask in After Effects by Kevin Snyder, who shows how to use this technique to facilitate more expressive animations with fluid and organic motion.
This technique has possibilities that extend much further than this single project and hopefully it will demystify many styles that are popular in motion graphics
Kevin uses two scripts over the course of the tutorial, using TrackerViz, which allows you to visualize positional data in After Effects, and Connect Vertex to Point, which will allow for the animation of individual vertices in an After Effects mask by attaching a vertex to any two-dimensional point property. Check out the tutorial for How to Use Mask Expressions on Individual Vertices for a More Dynamic Animation here.