In scuba diving, a freeflow occurs when the diving regulator continues to supply air instead of cutting off the supply when the diver stops inhaling, or starts to flow when out of the diver's mouth due to a pressure difference over the diaphragm or a bump to the purge button, and continues to flow due to the "venturi effect" of reduced internal pressure caused by high flow velocity of the escaping air. If the freeflow is caused by a "venturi effect", simply closing the mouthpiece over will stop it immediately.
Sometimes the freeflow will not stop when the backpressure is increased. This may be caused by very cold water freezing the first or second stage valve open, or a malfunction of either the first or second stages. If the freeflow is caused by freezing it will generally not be corrected except by closing the cylinder valve and allowing the ice to thaw, which requires an alternative air supply to breathe from while the valve is closed. As long as the freeflow continues, the refrigerating effect of the air expanding through the valves will keep the ice frozen, and air will continue to escape until either the cylinder valve is closed, or the cylinder is empty.
Other freeflows may be caused by the second stage valve jamming due to grit or corrosion products fouling the movement of the valve poppet, or the purge valve sticking in the depressed position. These can sometimes be stopped by pressing the purge button a few times to free up the works. If all else fails, the diver can breathe from a freeflowing demand valve by allowing excess air to ascape from the sides of the mouth and the exhaust valve, which may allow a safe ascent, or at least the use of as much remaining gas as possible.
If the diver can comfortably reach the cylinder valve, which is usually the case for side mount or sling cylinders, the cylinder valve can be opened and closed manually to control air flow while breathing during the ascent or exit, which will allow more of it to be breathed, and less wasted. This procedure is known as feather breathing.
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