This extension can unlock hidden features in the web version of X / Twitter. Before a new feature gets released to all users, it usually becomes accessible to the developers and testers first. This is done using "feature flags" - a mechanism that can turn certain features on or off for every account. "Secret Twitter Features" allows you to change those "flags" for your account, allowing you to try almost any feature that was added to the code. To change feature flags, find "Features" item in the left menu in X app.