What does the back button do that you didn't expect? When I click on a different page it takes me to that page, when I click back I go back.
From what I understand, breaking the back button would mean only the URL changes when you move pages, and back takes you off the domain entirely.
It breaks the state of the previous page. Do a search for something ("rust") go to an article, click back and you're search results are gone. If I'd done the same on a normal html page my search results would be there.