I am assuming that your question is about the knob and lever on the second stage.
The knob adjusts cracking pressure. It adjusts how hard you have to suck to get the reg to start to deliver gas. Common sense would denote that you'd want it set as sensitive as possible (all the way loose - counterclockwise), but then it will freeflow very easily. Tighten the knob up some (clockwise) to tighten the cracking pressure and keep it from freeflowing too easily. Most people have their primary set pretty sensitive and their backup ("octopus" on a standard recreational rig) pretty tight so it tends not to freeflow so easily.
The lever changes the backpressure inside the reg, controlling how the reg delivers air to you. With little backpressure (lever away from your lips), the reg will feel like it blows air into your mouth, like positive pressure. Push the lever toward your lips and it will give the reg some backpressure, requiring you to suck more to keep the flow of gas coming. The lever's effect is much more pronounced at 100 feet than at 10 because the gas is much more dense due to compression at depth. Common sense tells us that you'd want the lever all the way AWAY from your lips to provide an almost "power" breath (like the reg is breathing FOR you), but too much and the reg will freeflow just from normal breathing... Right in your mouth.
Think of the knob as an adjustment of how hard you have to suck to get the flow of gas to start, and think of the lever as an adjustment of how hard you have to suck to keep the flow of gas coming. Ultimately, you want them both as sensitive as possible, but you'll have to tune them down a bit to keep the regulator from freeflowing too easily and/or wasting breathing gas.
Most experienced divers "detune" their regs quite a bit... That is, they adjust them so that they don't get gas unless they suck purposely. They do this to prevent freeflow issues. Most beginning divers enjoy an easier-breathing, more sensitive regulator. With the lever and knob, you can choose whatever is more comfortable for you.
For what it's worth, these adjustments usually loosen somewhat as the regulator ages... That is, you may find yourself tightening things up progressively over many dives. I've also found that conditions tend to have a factor in these adjustments - gas density (air/"nitrox" vs. trimix), relative depth, and water temperature all have an effect on the adjustments, albeit only slightly. Consequently, feel free to adjust these "on the fly" and often to enhance your scuba experience.
Hope that helps.
