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.