Lynn has posted this concise and thoughtful introduction to writing flash at the recently started Texture Press wiki.

https://sites.google.com/site/texturepress/home/general-styles-and-genres-of-prose-and-poetry/flash-fiction-a-primer

(The wiki has just gotten started–but you can see where Texture is headed with it–a resource for writers, readers, with entries written with a bit of personality!)