| Hi Phoebus,
Yes, a majority of basses and guitars operate the best with a slight amount of "upbow" or "relief" in the neck. Reason being that strings vibrate in an elliptical pattern. So, a slight bow in the neck will follow the contour of the vibrating string whereas a flat neck would interfere with the elliptical contour, causing fret buzz.
Single action truss rods (ones that only bend the neck in one direction--away from the strings) are usually installed deep in the neck, (at least below the first half of the neck's thickness) with a large washer at the adjustment end and a "anchor nut" at the fixed end. When you tighten the adjustment nut, it pulls the washer and anchor nuts closer together, compressing the back of the neck.
Because the rod is installed deep in the neck, there is more wood above the rod to resist the compression than there is below the rod. Thus, only the back of the neck is compressed, causing the neck to "backbow", counteracting the pull of the strings.
Double action truss rods usually don't work by compressing the neck wood. Instead, they usually consist of a rod that is bent so that its two halves are next to each other (like a very skinny "u".)
Then, an adjustment nut is affixed to the top ends and, when tightened, causes the rod to move against itself, causing a bend. Turning the nut in one direction causes a backbow, turning it in the other causes an upbow. A double-action truss rod works by actually bending itself.
By the way, you could probably fix up the buzz on your bass by loosening your truss rod slightly (make sure the strings are tuned up to pitch when you loosen the rod). Start by only loosening it about 1/16 - 1/8 of a turn, then check the relief to see if you need to loosen it more.
If you have more questions about adjusting the rod on your bass, let me know. | 
