Current API doc states: "use a subselect query to load the additional collections"
To an SQL speaker, this could imply that the collection is being fetched in the same SQL statement that is selecting the parent.
SELECT ... from PARENT, (SELECT ... from CHILD)
Which is semantically equivalent to a JOIN, but is just a little confusing.
I think explaining in the API docs what sub-select actually does (a second select of all children from all parents via a sub-select including the original select statement) would help clarify.