Development
Read the README!
If you’re interested in developing plugins that ultilise the engines plugin, the first thing you need to read is the engines plugin README, which lays down all the basics about how it enhances Rails’ own plugin mechanism.
Tutorials
Gwyn Morfey has written a guide building plugins that can take advantage of the engines plugin’s features (link now dead, here’s the article at archive.org).
However, the best way to figure out what the plugin can do is to check the code itself - it’s fairly well documented!