Regulator free-flows are one of the greatest hazards when diving under ice or in very cold water (less than 4°C). The condition is caused by the sudden drop in pressure as air passes from the cylinder through the first stage. When high pressure air passes through the first stage, it hyper-cools the metal moving parts. In a piston reg, small ice crystals can block the piston open, causing more air flow and trapping the piston open, creating a vicious feedback loop of more air, more ice and a runaway free flow. The air pressure overwhelms the downstream valve in the second stage and all of a sudden you are receiving high pressure air right through the entire system, eventually rupturing the LP hose or damaging the second stage. This phenomena can also occur less commonly with diaphragm regulators. To minimize the likelihood of free flow, use equipment conforming to CE standard EN250 (Cold Water Use). Diaphragm regs are generally better than pistons, which allow water to enter the first stage. Diaphragms can also be fitted with cold water kits, reducing the cold water contact with metal, moving parts.
Procedures are critical too. Never inflate a dry suit, inflate your BCD and breathe in simultaneously. To decrease the volume of gas passing through the first stage, do these things independently. Heavy breathing or use of the purge button increases the cooling effect of the airflow, so try to avoid both.
Free flows can also occur at the second stage, usually on the surface and caused by low air temps and breathing a wet reg in the open air. To prevent this, never inhale on a second stage out of the water when you are in a cold environment. To begin your dive, inhale fresh air topside, then dip your head below water and exhale into the second stage. Do this two or three times to warm the second stage and then submerge and begin breathing in a controlled manner.
Ensure that the cylinder and the air within it is as dry as possible. Keep the system warm until the last moment prior to diving.
Between dives, ensure that no water enters the air intake of the first stage when drying the dust cap. If possible, dry the second stage fully before the next use.
Restrict yourself to no-stop dives at depths from which you can make a free ascent in an emergency or carry a redundant tank and regulator, ensuring that you will be able to turn off the valve of the free-flowing tank quickly. Make sure you have practiced using your bailout. Free flows are extremely chilling and you want to switch to bailout as soon as possible.