system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a web page that takes too long to load suddenly pop up for a split second when I hit the back button?

It's happened to all of us.
gpt
It's a setting in your browser that prevents the page from displaying until enough has been downloaded to cache or else images like banners would pop in after text has loaded.  This is undesirable because it could cause the text to move around while your browser suddenly makes space for the image.  A good visual would be to imagine copy-pasting a picture into a Word document. 

You can actually go into a browser like Firefox and turn this feature off to see the page elements render as soon as they are downloaded, but then pages that load slowly can be unusable as clickable elements get shoved around as room is made for things like images or flash windows.

When you press the Back button, the flash of the webpage that you see is probably the portion of the page that has been cached.

tl;dr: The browser is waiting for all the pieces of the page to download before lifting the curtain and showing you the whole page in all its glory.  When you press 'back' the curtain flickers and you are seeing the elements of the incomplete page.

[edit: Forgot to add in the part about the Back button]