How does the Quasar know when it should defer execution of something that might block? Does the user declare this in some fashion? In the article, it looks like this is done with try-with-resource blocks.
Quasar identifies blocking methods if they declare that the throw a SuspendExecution exception, marked with a @Suspendable annotation, or listed in an external file. Pulsar, Quasar's Clojure API, marks suspendable functions differently, but that's an implementation detail.