This is a spin-off of Stackless, a version of CPython that supports micro-threads called "tasklets".
Tasklets run pseudo-concurrently (typically in a single or a few OS-level threads) and are synchronized with data exchanges on "channels".
A "greenlet", on the other hand, is a still more primitive notion of micro-thread with no implicit scheduling; coroutines, in other words.
This is useful when developers desire to control exactly when their code runs.
They can build custom scheduled micro-threads on top of greenlet.