It's about pressure. Boyle's law, if you think about it. Pressure is inversely proportional to volume.
As you inhale, your ribcage expands. It's volume becomes larger, hence the pressure in the lungs (or for that matter the thoracic cavity) becomes lower relative to the external environment. Air will flow from regions of high pressure to low pressure. This drives everything from weather systems to breathing.
The air on the outside of the body is now at a higher pressure relative to the air inside the chest. So air flows through the pharynx, larynx, trachea, bronchi, broncioles into the alveoli, filling the lungs. Until the respiratory muscles relax, and passive elastic recoil causes the size of the thoracic cavity to shrink, pressure in the chest to go up, and air to be exhaled to the now relatively lower pressure external environment.
Now that's in the normal situation. Of course, if you then go and punch a hole in the rib cage into the pleural space or further, when the pressure in the chest goes down, air has two ways to flow into the thoracic cage. It has the nice / good pharnyx/larynx/trachea/bronchi/bronchioles/alveoli that gets pulmonary gas exchange going and keeps us happy and pink, and it has the easier option of just going through the big hole in the chest, where it's highly unlikely to end up too close to too many respiratory exchange surfaces. Generally, the larger the hole in the chest, and the more air moving through it, the greater the problem.
So by sealing off the sucking chest wound, we're getting rid of this alternate route for air to get into the thoracic cavity. This is good.
The problem comes when the knife, bullet, fencepost, crossbow, lamprey, or swordfish has also punched a nice hole into the lung. If we now close off the surface of the chest, we have the potential for a pneumothorax to form. When we exhale, and the pressure in the chest goes up, there's a chance that some of that air is going to go through the hole in the lung, and end up in the pleural space or the mediastinum. This can potentially lead to a pneumothorax.
The idea behind a three-way dressing is that it should open when the patient exhales, preventing air from accumulating in the pleural space, by allowing it to leave but close when the patient inhales, preventing air from entering the pleural space from outside the body.
Patients with a sucking chest wound are going to end up with a chest tube in the ER. The idea behind burping the wound is simply that if you're going to decompress to relieve a tension, or you suspect the patient is developing a pneumothorax, if there's already a great big hole in the chest that you've covered up -- perhaps opening this hole is going to allow you to vent the extra pressure. There's definitely a potential for the wound to clot. It's going to depend on the size.
A lot of people just cover the wound on all four size out of habit, and get away with it because the chance of developing a tension pneumo in the relatively short transport times most EMS providers have is fairly small. Covering the wound also becomes less important if you have the patient intubated, and you're doing PPV, in contrast to the body's physiologic negative-pressure ventilation.
