A card can have the navigation focus. There is only ever one card with the focus, and it is highlighted by a flashing colour bar. When you create a new card with Ctrl-N or the New Card command, it is created after the focus card in an available column or in a newly created column.
You can navigate around the storylines window with the keyboard arrow keys, using Alt-Left, Alt-Right, Alt-Up, and Alt-Down. The current focus will be set to the first card in the specified direction. Alt-Home and Alt-End will take you to the start and the end of the storylines window respectively.
You can insert a column in front of the focus card with Alt-Insert, and delete the focus card using Alt-Delete.