The user discovers it because it is practically forced on them. It is awful UI.

When a user wants to return to the navigation bar at the top he scrolls up. The navigation bar then immediately gets nearer.

The user discovery happens because the act he performs provides the exact intent you need to give him the shortcut.

Also for clarity this is only relevant for content based sites and not apps. It is vanishingly rare for users to scroll up when reading content unless they want to reach the top

	▲	Night_Thastus 2 hours ago \| parent \| next [-]
		>It is vanishingly rare for users to scroll up when reading content unless they want to reach the top This assumption is the problem. No, it is not rare for users to scroll up while reading. People are not perfect machines that read everything in one pass and understand it fully. They may go back to re-read, or look at an earlier image or figure in the text, or otherwise. Sometimes people zone out for a minute and find they 'read' with their eyes but didn't actually take in the content. That requires going back. For me, scrolling up to re-read is a basic use case of a web page. If it can't do that properly, it has failed.
	▲	oe 3 hours ago \| parent \| prev [-]
		That’s not why user scrolls up, or at least not the only reason. For example, reading this discussion I constantly scroll up and down to center the text on screen. If the header only appears after scrolling up for a bit then it’s not so bad, but most implementations show the header after scrolling 1px up. That’s infuriating.