A DV is basically a small chamber filled with air. It has three openings:
1. The mouthpiece - air in and out
2. The exhaust port - air out
3. The direct feed - air in.
It also has a flexible bit - the diaphragm.
When you breathe in, you pull air through the mouthpiece. Doing this decreases the amount of air in the chamber. The diaphragm is therefore sucked inwards.
Resting against the diaphragm is a lever. When the diaphragm is pulled inwards, it presses against the lever. The lever is therefore also pulled inwards. The lever moves the inlet valve, which allows air to enter from the direct feed. Air rushes in, the chamber fills up with air again, the diaphragm is no longer being sucked inwards, so returns to it’s original position. Because it’s on a spring, the lever is also pushed back. It is therefore no longer pulling the inlet valve away from the direct feed, and so the direct feed is blocked off. Air stops entering the DV.
And then you breathe out. There is now more air in the DV chamber than it wants to hold, so the exhaust valve opens and the air rushes out.
Basically, the DV ensures that, whether you’re breathing in or out, it is always full of air.
But what about the extra control bits? I hear you cry.
There are usually two things you can change on a DV: Cracking pressure, and Venturi resistance.
The cracking pressure is the amount of effort it takes to make your DV start giving you air. The lower this is, the easier it is to breathe. The higher it is, the less likely a freeflow is too happen.
The cracking pressure is controlled very simply. The lever that is moved by the diaphragm to open the inlet valve works against a spring. By changing the amount of resistance this spring offers, you change the cracking pressure. And to change the amount of resistance, you simply wind the spring in or out, by moving the cracking knob.
This is a DV with maximum cracking pressure
This is a DV with minimum cracking pressure
As you can see, the spring is more tightly wound in the DV set to maximum cracking pressure.
The Venturi lever is something else you can adjust to decrease the chances of a freeflow. Basically, when air enters your DV, it's aimed at your mouth, to ensure the best airflow.
Air enters your DV through a fairly small hole. The Venturi lever simply moves a small piece of plastic above the hole. Then, when air enters your DV, instead of rushing straight into your mouth, it is directed to push against the diaphragm of the DV. This significantly decreases the chances of a freeflow, but only causes a marginal increase in breathing effort.
