This article is neither new nor my own, but I've shown it to a few people lately and despite their having worked with SSIS for months, none of them had seen it.
This is a white paper that describes how the SSIS run time engine and data flow engine work, and how to take advantage of that knowledge to build better performing SSIS packages. It has excellent coverage of execution trees and memory buffers, and how they are used "under the hoods" in your data flow.
If you haven't read it before, please check it out today and tell a friend. It's required reading for anyone building SSIS packages in the real world.