If you're already inside the TDD, you'll end up using the ⌘ + u shortcut to run all the tests, hundreds of times every day. But as your test base starts to grow, running all the tests can take a long time. So, you may start to have the habit to run only the test you selected with the cursor, using the mouse and the checkbox next to the name of the test; this, however, means you break your flow to move the mouse there.
There is a shortcut for running only the current test: alt + ⌘ + u. It's just a shortcut, but knowing it will increase your productivity.