Clicking the macro with the shift or ctrl key down will set your current target as your focus and cast Shackle. You can then change targets and clicking the macro (without shift or ctrl press) will recast Shackle on your focus. If you don't have a focus, clicking the button will cast Shackle on your current target.
Type /shw or /sheepwatch to show the config UI.
1.0 - Initial Version
1.1 - Updated toc; added some more smarts to handle focus vs target casting