"Blocking (on IO)" in OS jargon means that the thread can yield the CPU to other threads because it's otherwise waiting for some external event (e.g. some IO op

"Blocking (on IO)" in OS jargon means that the thread can yield the CPU to other threads because it's otherwise waiting for some external event (e.g. some IO operation to complete). When the IO operation is ready the thread is then marked to be awakened and the scheduler will context switch back to it whenever it sees fit (given other considerations like thread priority, etc). From the POV of the thread that blocks nothing really happens while it's waiting. Once it awakens it has the result of the operation (or an error) and it can observe that more time has passed by inspecting the timer. For all intents and purposes the operation was "synchronous" from the POV of the thread.