I'm not expert Clojurist, but my first guess of this is that it prints and pushes every non-nil value into a go channel xf, one-by-one, recursively, in a new th

I'm not expert Clojurist, but my first guess of this is that it prints and pushes every non-nil value into a go channel xf, one-by-one, recursively, in a new thread, then return the channel, presumably so more work can be done and passed to it. The number of exclamations in <!! means something, but I forget what, I think something about non/blocking.