If you are the proxy for another user, switching to that user is now much easier.
A “Log In As Proxy” link has been added to the top navigation bar; clicking that link will display users that you are a proxy for. Clicking a user’s name logs you in as that user, and you’ll see their name displayed next to the “Welcome” in the navigation bar.
You can easily switch back to your user, too. When you are acting as a proxy for another user, the link next to their name changes to “Return To Your Account”.
For a quick tutorial, see this article.
The old way of logging in as a user you proxy (their username, your password) has been deprecated, and will be removed in the coming months.