Sorry if this is a bit late, but it's a Nginx Reverse Proxy. Basicaly you get another server and redirect your main web server through there. While doing that you can serve a HTML file.
I'm guessing there are a lot of ways to do this, but this is one way.
Buying
Create a loading page
Submitted by The Plug, 12-07-2016, 05:15 PM, Thread ID: 22350
Thread Closed
RE: Create a loading page 29-07-2016, 04:12 PM #18
29-07-2016, 04:12 PM
#18