It is evident to me that divers rarely use the sensitivity knob and venturi assist feature of their second stage regulator . Sensitivity (or adjustment) knobs are designed to adjust the inhalation effort and intensity of air delivered from your second stage. In short, if you are diving in calm conditions with no current then you will have your sensitivity knob set to your preferred setting. If you are finning against current causing your breathing rate to increase... then you can wind the sensitivity knob all the way out which will deliver a higher intensity of air.
When sensitivity knobs are not used or cleaned the threads get a build up of salt and particular making it much harder to turn and eventually it will ceaze completely. It is easy to prevent this from happening and to do so all you have to do is whenever you wash your regulators in fresh water, submerge the second stage (being careful not to depress the purge button) and wind the adjustment screw all the way in and all the way out a few times. This prevents salt and particular from building up in the thread.
Another feature of a second stage is the venturi assist leaver. Noting the sensitivity knob adjusts the intensity of air delivered through the second stage... the venturi assist leaver affects the direction of air flow within your second stage. A venturi assist is usually labelled as +/- or dive/pre-dive. If you purge your second stage with the venturi lever set to -/pre-dive it should not free flow. If you purge your second stage with the venturi lever set to +/dive then it will generally free flow. This does not mean there is anything wrong with your second stage, it means the venturi is working as it should. To stop a free flow simply block the mouthpiece with your hand or set the venturi leaver back to -/pre-dive.
If you have the venturi assist feature, it is also good to rotate it back and forth while washing your regs submerged in fresh water preventing sand, salt and particular being caught in it's o-ring. You will be surprised how this little action helps the performance of your regulator over time...
The best way to understand the function of these features is next time you go for a dive, take the time to stop what your doing and operate the venturi assist lever back and forth. You will feel the difference between the +/dive and -/pre-dive setting (it is much easier to inhale on the +/dive setting). Then leaving your venturi leaver set to +/dive, wind your sensitivity knob all the way in and all the way out. You will notice the difference in the intensity of your air delivered. When your happy set the sensitivity knob to your preferred setting and enjoy the rest of your dive!