How do I simulate offline in Chrome with a delay so it won't take effect immediately

Question

If I turn on offline in throttling in Network DevTools, it will take effect immediately. But I hope I can simulate offline with a delay, i.e. when I open my webapp the initial web requests succeed, then network goes down because this is more like what happened in my client situation.

Then I didn't know what I had done I somehow succeeded in doing that. The offline kicks in after my webapp made the first couple of network requests and made websocket connection, then network went down. But after I turn off offline setting I can't reproduce anymore.

So how do I do that ?