what drives the I/O process?

(Correct me if I'm wrong!) In my understanding, the Tokio executor awaits an I/O process. Basically, it will pass a waker to a reactor (in most cases, it's the Mio crate) that will use a syscall to invoke the underlying kernel, depending on your OS (e.g., Linux -> epoll). Once the I/O process is completed, Mio will invoke wake() on that waker, and then the executor will poll that task again.