No Documentation for `deferred Fragments` or `@defer`

In the Documentation here https://relay.dev/docs/tutorial/queries-2/ , if you expand the "Deep dive: When to use a secondary query" (nice addition btw!), it mentions "deferred Fragments" and that they will be explained "later".

But I couldn't find any explanation on the topic anywhere else :/
There is a Glossary entry for `@defer`, but it does not explain anything really.

The relay compiler also doesn't recognise `@defer` out of the box.

What is the state of the deferred fragments feature?