This allows to create IN queries like WHERE column IN (SELECT otherColumn FROM ...).

It can be done by using the filterBy() methods:

$innerQuery = BookQuery::create()
  ->filterByTitle('%Galaxy%', Criteria::LIKE)
  ->select('author_id');
AuthorQuery::create()
  ->filterById($innerQuery)
  ->find();

which will generate a statement like:

SELECT * 
FROM author 
WHERE id IN (
  SELECT author_id
  FROM book
  WHERE book.title LIKE '%Galaxy%'
)

For known relations, there are useInQuery() methods which work similar as the useQuery() methods:

AuthorQuery::create()
  ->useInQuery('Book')
    ->filterByTitle('%Galaxy%', Criteria::LIKE)
  ->endUse()
  ->find();

Propel creates convenience methods for existing relations on query classes like AuthorQuery::useInBookQuery(), which is easier to use than useInQuery('Book').

NOT IN queries are created by passing CRITERIA::NOT_IN to filterBy() or the useNotInQuery() method.

I have build a similar thing for EXISTS, so this is mostly just abstracting from that.