See https://github.com/scalaz/scalaz-stream/issues/274, specifically the comment below from djspiewak.
"From a conceptual level, the problem here is the interface point between the "pull" model of Process and the "push" model that is required for any concurrent stream merging. Both wye and njoin sit at this boundary point and "cheat" by actively pulling on their source processes to fill an inbound queue, pushing results into an outbound queue pending the pull on the output process. (obviously, both wye and njoin make their inbound queues implicit via Actor) For the most part, this works extremely well and it preserves most of the properties that users care about (e.g. propagation of termination, back pressure, etc)."
The second parameter to njoined, maxQueued, bounds the amount of prefetching. If that parameter is 0, there is no limit on the queue size, and thus no limit on the prefetching. The docs for mergeN, which calls njoin explain a bit more the reasoning for this prefetching behavior. "Internally mergeN keeps small buffer that reads ahead up to n values of A where n equals to number of active source streams. That does not mean that every source process is consulted in this read-ahead cache, it just tries to be as much fair as possible when processes provide their A on almost the same speed." So it seems that the njoin is dealing with the problem of what happens when all the sources provide a value at nearly the same time, but it's trying to prevent any one of those joined streams from crowding out slower streams.
