How do you adjust preload on shocks?

I have a Trek 3900 Disc Mountain bike and i was wondering how to adjust preload on the shocks/fork. The shocks are 100mm Travel w/ 30mm stanchions SR Suntour XCM and are also coil springs with the preload adjustment. I was wondering if turning the knobs that have + and - do anything to how much i absorb when going off dirt jumps etc. and how to do it.