Grimace is an outcome of a research project about the relationship between emotions and different media types, and how this relationship can inform the design of interactive systems.
You can download the documentation that was created as a companion to Grimace. It covers the theory behind it and goes into detail on the technical implementation.
Oliver wrote his master's thesis about this topic.
Prior to the release, Thomas conducted an online study which tested how well users could interpret the emotions displayed by Grimace.