The receptionist hands out clipboards; she never stands watching someone fill one in.

One thread runs your JavaScript, but most of a server's time is spent waiting on sockets and disks. That waiting is handed to the operating system and some helper threads. The JS thread only runs a short callback when a result arrives, so thousands of idle connections cost almost nothing. The analogy breaks if the receptionist starts typing a long report herself: see the third stop.

Read it once, then a question tests whether the trick stuck.