They're not really that interesting. They're "reduce transformers". So, take a reduction operation, turn it into an object, define a way to convert one reduction operation into another and you're basically done. 99% of the time they're basically mapcat.

The real thing to learn is how to express things in terms of reduce. Once you've understood that, just take a look at e.g. the map and filter transducers and it should be pretty obvious. But it doesn't work until you've grasped the fundamentals.

Canonical example is rewriting a non transducing set of collection transformations like

   (->> posts
      (map with-user)
      (filter authorized?)
      (map with-friends)
      (into []))

    (into []
          (comp
            (map with-user)
            (filter authorized?)
            (map with-friends))
          posts)

map for example is called with one arg, this means it will return a transducer, unlike in the first example when it has a second argument, the coll posts, so immediately runs over that and returns a new coll.

The composed transducer returned by comp is passed to into as the second of three arguments. In three argument form, into applies the transducer to each item in coll, the third argument. In two argument form, as in the first example, it just puts coll into the first argument (also a coll).