You can now return an action along with your card-back-section in order to show a button at the top right of your section.
An action consists of two required properties:
text - The text to show on the button
callback - The function to be called when the user clicks the button
Your callback can do anything a card-button callback can do, such as opening a t.popup or a t.modal for example.
Check out the full docs for the card-back-section capability for more information: card-back-section