By focused I meant that the cursor is in the field (allowing you to type in the field).
Really with this option turn on we are just defaulting to what ever the browser's implementation of the HTML5 spec is, so for example when enter is pressed with the dropdown select list focused then the spec says that the key event will select the currently active <option> and won't submit the form. Likewise for a textarea, enter is going to add a new line to the text you entered into the textarea.
The main reason this option was added was to give people the option to stop form submissions when the enter key was pressed, rather than globally enabling it on all of the form fields. I've updated the wiki and tooltip text to explain that.