Since Kotlin/Native enforces different semantics between its main thread and background threads, it's only logical to run the tests once on the main thread and once on a background thread to ensure compliance. This function accepts a lambda which it runs synchronously (which will be on the main thread) and then transfers that lambda to a background thread where it's run a second time. With the constraints of its memory model being runtime checked, running your unit tests on both the main thread and a background thread prevent bugs like the one which occurred with ArrayDeque .