Well, the original idea was to have a demo or tutorial on how to make a missile launcher that extends from an enclosure and then points at a target by using a single input channel. The problem is that when trying to have tracking work as part of the rig action, it all breaks down and stops working correctly.
The video shows the whole process if you want to follow along to what I was doing and what the general idea is. Problem happens somewhere in the last 5 minutes. Unfortunately I can’t offer the .blend file as it was at the last stage. (I sorta rage-quit after ending the video recording. Sorry.) :o
I’m almost thinking I messed up by using empties to pass data from an action. Perhaps it would have worked if I used bones for that instead?
---- Edit ----
I had the answer right in what I was thinking at the end. Needed an (empty) bone to pass a value to the tracking constraint. All good now. 
Time to post the good video under the tutorials section…