This is taking advantage of coroutines and to-be-closed variables.

To-be-closed variables were recently introduced in Lua 5.4. Lua runs a destructor method when these variables go out of scope, like the RAII pattern in c++.

Coroutines have been in Lua for quite a while now. They allow cooperative concurrency (as opposed to preemptive concurrency a-la threads). It works similar to async generators as found in Python, C#, etc, except that Lua coroutines are stackful. You can yield inside an inner function and it'll pause the whole call stack. In the article, the yield is inside the async_sleep function. When you call a function that yields, you don't need to explicitly mark that with an "await".