Questions to all the owners of it.  What is that black strap about 6 inches long for?

The short strap allows you to quickly move your weapon from the chest position to your side by pulling it.

Check out the one point sling instructions on the Troy website.

I must have it installed wrong, because the strap is behind my back.

here is a picture of the sling.  As you can see, on the far right of the pad, there is a buckle.  That's where the 6inch long strap is located at.  

Quoted: I must have it installed wrong, because the strap is behind my back.

Troy likes users to sling it over their weak side shoulder, contrary to what a lot of shooters are used to.   So if you do it the common way, it will be in back.   I'm just about ready to cut my damn strap off, as I have no use for it and its coming apart by itself due to their crappy stitch job:

I shoot my rifles right handed.  I put it over the weak shoulder.  I have since moved the buckle with the small strap, near the center of the sling.  This prevents the strap holding the rifle to slide around.  I dont know if that's how it was meant to be used for sure though.
The rifle sits under my right arm pit.  Works for me.  I can wear the shoulder pad a little off, so the rifle sides in front of my body. Then I can just pull the strap and rifle will be under armpit.  I prefer not to have my nuts crack, so I think i may cut the strap and just have it sit under my armpit.