Originally Posted by Landgolier

The shifter (as in the part on the handlebar) pulls exactly the right amount of cable to move the RD 1 cog's worth for every click. The only tension on this cable is what the spring in the RD provides. The barrel adjuster just lengthens or shortens the amount of cable in between the shifter and the RD; you're not adjusting tension at all, just location. "Tighter" and "looser" seem like appropriate terms because you're turning threaded gizmos and one direction is against resistance and the other isn't, but it confuses the issue by making you think you're playing with cable tension and not length.