It’s called the /ae/ split, and the system Philadelphians use is different from most of the rest of the country — that’s known as a nasal system. It’s why you switch up words like gas (to almost like gaeh-us) and "manage" (almost like mahn-ij) when putting on your best Philly accent. What Sneller found was that there was a shift happening in the /ae/ split, and she wanted to focus on who was driving those changes.