Ok, I think you understand, but here's a slightly clearer explanation. 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. The goal is simply to align the places where the RD stops with the positions of the cogs. Again, this is a pretty fine adjustment, 1/2 a turn can be the difference between good and useless, and 1/4 turn is the difference between OK and awesome. Complicating this is the fact that the upper pulley on most RD's "floats" a little to compensate for slight misadjustment. It's also possible to screw the barrel adjusters down so far that you can't even get into the top or bottom gear, the shifter just doesn't have the cable to give you or can't pull another full interval as the case may be.