Cmd-↓ in the Finder is equivalent to Cmd-O. Holding down Option at the same time will instruct Finder to close the window afterwards.*
It looks as though it was a good call by @ccstone without much to go on.
@iamdannywyatt, this is explained under:
How Do I Get The Best Answer in the Shortest Time to Questions on the KM Forum? as linked from the pinned post Getting Started with Keyboard Maestro and the Forum.
Good luck and let us know how you get on.
* If you select a file, hold down the option key and look under "File" again, you will see that "Open" has been replaced with "Open and close window".