Committing here a patch by Bastien Montagne (mont29), a more understandable Translation Constraint UI.
Before: http://www.pasteall.org/pic/12578
Now http://www.pasteall.org/pic/12258
From the description:
"When you set “X” under the Destination’s “Z”, it does not mean that the Z transform of the source should affect the X transform of the destination, but rather that the X transform of the source should affect the Z transform of the destination…"
The new UI should make it a bit more clear.