The Elastic Racetrack - Flash player mental model
This is one of the most valuable articles for a Flex developer.
It is an update for Ted Patrick's original article of 2005, and explains how flash player executes frames.
This article clarifies a lot when it comes to building custom components (why should the invalidation/validation model be used), performance and lots more.
These are links to excellent videos regarding creating custom components, which refer to the elastic racetrack model:
Building custom components (ppt, source, video)
Diving deep with Flex component lifecycle