Links to a detailed  technical explanation of GDV:
http://www.bmdinfo.org/Health/Gastric_Dilatation_Volvulus_Bloat-Bernese_Mountain_Dog.php
https://veterinarypartner.vin.com/default.aspx?pid=19239&id=
Acronym to know  – GDV (gastric dilation and volvulus)

What is torsion:
Gas starts to build up very quickly in the stomach, as the stomach expands rapidly, putting painful pressure on other surrounding structures, such as the vena cava
Blood supply is cut off to vital organs, and especially the stomach. It turns on its axis (torsion) (twisting like a balloon) and tissue begins to die off
The spleen sometimes twists along with it
Usually happens to larger, barrel-chested breeds, and for reasons not understood, seems to be more common with males
No warning and critical condition

Symptoms:
Extremely painful, restless, voraciously eating grass, or seem “off”
Restless activity, obvious discomfort, and hard feeling abdomen, often with a larger appearing abdomen
Death follows very quickly, usually within two hours, generally if not treated immediately

Treatment:
Must get the air and stomach contents out (decompression) quickly, and usually surgical intervention
During surgery, the stomach is tacked down to the posterior wall of the abdominal cavity so that it won’t twist again
Bloat can still occur

The Twisted Truth: Bloat Could Happen to Your Dog
Updated: Wednesday, 10 Aug 2011, 9:36 AM CDT
Published : Wednesday, 10 Aug 2011, 9:36 AM CDT

Seemingly out of nowhere, your dog could become affected by bloat (correctly called gastric dilatation and volvulus, or GDV). In less than 1 hour from onset, your canine companion could be facing a life threatening crisis with bloat. Every minute is crucial with this condition in recognizing the signs and seeking emergency treatment. It is truly the “mother of all emergencies”. Bloat or dilatation is the initial phase of GDV with stomach distension. As GDV develops into “volvulus” or torsion, the stomach twists on itself which traps food, air and water within the stomach, quickly leading to a rapid onset of serious problems. The bloated stomach interrupts blood flow within the abdomen and stomach tissue, leading to shock and injury to internal organs. The combined effect of all these developments can quickly become fatal for your pet.

Is your dog at risk:
GDV classically affects large breed, deep chest dogs including German shepherds dogs, Great Danes, standard poodles, Dobermans, and Weimaraners (to name a few) but it can be seen in all breeds. The classic onset can follow an event such as a recent meal or drinking followed by exercise, play or stress. Genetics are felt to play a large role in an individual dog’s risk of developing GDV, particularly if a first degree relative has been affected. Potential additional risk factors also include feeding only 1 meal per day, eating too rapidly and drinking excessively (resulting in “gulping” air).

How do you know if your pet’s suffering from GDV:
Some common signs of bloat include restlessness, pacing, salivation, retching, unproductive attempts to vomit, and distension of the abdomen, though abdominal distension may not be seen in some of the large deep chested dogs. Seek veterinary attention immediately if your dog is showing any signs that may be consistent with GDV. Early recognition and treatment are the keys to surviving this condition. Don’t worry about over-reacting; it’s better to be safe than sorry!

Emergency treatment:
A dog suffering from GDV needs emergent medical attention. Delay in diagnosis and care can result in serious or fatal complications. If you present your pet to the veterinary clinic with signs consistent with GDV, they will likely initiate testing and treatments right away. Your pet will have x-rays of the stomach to officially diagnose the GDV condition, but consistent exam findings and symptoms are enough for presumptive diagnosis often times, and your vet will likely begin some initial stabilization treatment. Again, every minute counts with GDV. One of the first interventions that your veterinarian will administer to your pet is intravenous (IV) fluids. Another imperative early treatment is stomach decompression. Decompression reduces distension and stretching of the stomach tissue. Decompression can be done with a trocar, or needle, placed into the stomach from side of abdomen, but may also include a stomach tube, placed down the throat to the stomach with sedation or anesthesia. Once the dog is stabilized, emergency surgery will be recommended to allow for correction of stomach positioning. A gastropexy, or tacking, will be performed to hold the stomach in its normal position to prevent future GDV.
The more you know about bloat, the better you can monitor your pet for problems. Be prepared and know where to take your pet in the event of an emergency; have a plan! Talk with your veterinarian about your pet’s particular risks for developing GDV. In breeds with a high risk of bloat, there is a preventive surgery called a prophylactic gastropexy that can often be performed to prevent stomach rotation. Ask your veterinarian for details and advice if you would like to discuss preventive surgery for bloat. For further information and facts on GDV, check out  https://veterinarypartner.vin.com/default.aspx?pid=19239&id= .