Preload Adjuster Use on SR Suntour XCM I've got an SR Suntour XCM HLO 27.5" 100mm travel that comes with my bike. It has preload adjuster on the left and lock on the right.

What I wonder is how to properly adjust the fork softness/stiffness as I encountered some bumps with stiff feelings on the fork.

Also, if I lock it and turn the preload adjuster, I can turn it more than if I let the fork open. What causes this and should I lock it before adjusting the preload?
Turning the preloaded dial towards the negative symbol will decrease the preload on the spring. Therefore if you find the forks are too stiff, backing off the preload will soften the fork. Alternatively if you wish to firm up the fork, turn the preload dial towards the positive symbol. This will compress the spring and stiffen up the suspension. 

I would adjust the preload with the lock out off, though I don't believe it would make any difference beside allowing you to compress the fork as you dial preload on/off to see what affect it has on the fork. 

Hope that helps!