Air sacs are located inside the neck, chest and belly. When ruptured, air will leak from the sac and accumulate under the bird's skin. If air is not released, the tear in the sac will enlarge.
The vet or you can disinfect the ruptured skin and with a sterile needle gently prick the area to allow the air to escape.
Repeat if necessary (surgical repair / antibiotic therapy may be necessary).
Budgerigars may rupture their air sacs by flying into windows or the result of a heavy landing.
A startled bird due to night fright bird may damage their air sacs and do so much more damage so be prepared and always have a low wattage night light for your bird/s.
A swollen area along the breast which when held feels very spongy and may even emit a crackling noise when touched is certainly an air sac rupture
Gas under the skin or Subcutaneous Emphysema as it is normally called is the air that penetrates the subcutaneous tissues as the result of damage to part of the respiratory system.
This can be alarming for the owner when the bird blows up into a grotesque shape within a few hours.
Once access of air is stopped, however, the gases are slowly absorbed.
This cock suffers from an air sac rupture but at the time this picture was taken he just appears to be fat, on another day he can be twice as bloated.
Source: Avian Web