Leiningen is very small. The latest release is only around a couple thousand lines of Clojure; you could probably read through the whole project in an hour.
When you launch Leiningen, it must start an instance of Clojure to load itself. But this instance must not affect the project that you're building. It may use a different version of Clojure from Leiningen, and the project should be in a fresh JVM. Leiningen uses ant's java task to fork off a separate process for this purpose. The leiningen.compile namespace implements this; specifically the eval-in-project function. Any code that must execute within the context of the project (AOT compilation, test runs) needs to go through this function.
The exception to this rule is when :eval-in-leiningen in project.clj is true, as is commonly used for Leiningen plugins.
TODO: what goes where? a tour through the launching of a task
Leiningen is extensible; you can define new tasks in plugins. Add your plugin as a dev-dependency of your project, and you'll be able to call lein $YOUR_COMMAND. See the plugins guide for details.
The mailing list and the leiningen or clojure channels on Freenode are the best places to bring up questions or suggestions. If you're planning on adding a feature or fixing a nontrivial bug, please discuss it first to avoid duplicating effort. If you haven't discussed it on the mailing list, please include in your pull request details of what problem your patch intends to solve as well as the approach you took.
Contributions are preferred as either Github pull requests or using "git format-patch" and the mailing list as is requested for contributing to Clojure itself. Please use standard indentation with no tabs, trailing whitespace, or lines longer than 80 columns. See this post on submitting good patches for some tips. If you've got some time on your hands, reading this style guide wouldn't hurt either.
See the complete list of known issues.
TODO: integrate with plugin guide
clj
(apply + (for [d (tentacles.repos/downloads "technomancy" "leiningen")
:when (re-find #"\d\.\d\.\d-standalone.jar" (:name d))]
(:download_count d)))