This information might already be known ... I've seen pieces of it in various places, but I figured I would test the item and post the results here, in case it is helpful. (This is for the version 3.7)
I did the testing by attacking the kobolds in Anslog's Compass, with a plain hatchet, on a Ranger with 10 Might, a reasonable number of times.
Three strange properties of the Stalker's Torc:
1) Unfavourable Property - If you attack the same creature that is engaged BY your Animal Companion, you do not get the 20% damage bonus
2) Favourable Property #1 - If you attack a creature that is NOT engaged BY your Animal Companion, you do get a 20% damage bonus
3) Favourable Property #2 - If you remove the Stalker's Torc, properties '1' and '2' above are not changed
As a note, the way I determined this (in case there is something wrong with my method!), is that I attacked the guys a number of times and noted the damage. Under the scenario where I was attacking the same thing as my Animal Companion, I always did between 9 and 13 damage. When I attacked someone not attacked by the Animal Companion, I was doing up to 15.4 damage. Similarly, when I removed the Torc and attacked someone not attacked by my Animal Companion, I was still doing up to 15.4 damage.
(That was the extent of my testing, so if the Torc is really doing the "20% damage over 3.0 seconds" as a DoT that doesn't appear in the log, I did not test for that).
