Quote Originally Posted by Shady_arc View Post
for describing a destination of movement, "за" takes a noun in Accusative when you are describing motion with a goal to get behind something. When you are simply telling the position behind some object, it takes Instrumental.
Yes -- and you can generalize this for certain other prepositions like над ("over") and под ("under") -- instrumental for static position, accusative for motion.

Also, note:

Он сидел за столом. (He was sitting at [lit. "behind"] the table.)
Он сел за стол. (He sat down at the table.)

The latter uses the accusative because "to sit down" implies movement, even though it's not one of the "Verbs Of Motion".