If you have the stock springs on the bike, then you really need to get the correct rate springs on there. No amount of knob twisting will get it set up very well for you.
Turning an adjuster in (clockwise) creates restriction (damping). Turning it out (counterclockwise) decreases damping. These adjusters will mosly affect the low speed damping (not so much on roots, rocks, and ruts - high speed stuff), as it relates to the speed the shock or fork travels within a certain distance / time. For example, landing from a jump is a low speed compression, and hitting a 6" log head on is a high speed compression hit.
The shock's rebound adjustment will have a slight effect on the compression as well due to the internal structure.
:) So like I was saying, get the correct rate springs.