ITS VERY ANNOYING WHEN, just as you are about to start a dive, your regulator suddenly and uncontrollably gushes air. Those who dive in cold fresh water can experience a similar effect while under water, and this can even be life-threatening if air reserves are low.
Why does it happen?
Free-flows due to regulator freeze-up are caused primarily when ice forming around the mechanicals of a regulator first stage prevents it from functioning properly. The inter-stage pressure - that is, the pressure of gas arriving at the second stage - is greater than the second-stage valve spring can handle, and the gas forces its way through.
Icing of the second-stage valve can have a similar effect in that the mechanism can get jammed with ice and the gas flow is unregulated. However, this does not explain why your regulator goes out of control just at that moment when you are about to enter a warm tropical sea.
Assuming that the first stage of a regulator is working properly, air is delivered to the second stage at 8-10 bar more than ambient pressure.
The pressure-sensing diaphragm at the front of the regulator (it also doubles as a purge button) presses down via a lever and opens the valve, so that air at a matching pressure enters the regulator body and allows you to inhale.
It should be able to give you exactly the amount of air you demand, neither more nor less.
Regulator designers strive to give you a device that makes inhalation as effortless as possible. To make the flow of air through the body of the second stage clean and uninterrupted, they try to design in a venturi effect.
In typical designs, this results in a very clean flow of air rushing past the back of the pressure-sensing diaphragm.
But a sudden rush of fast-flowing air can cause an apparent drop in pressure behind the diaphragm. The diaphragm in turn is pushed in to compensate, opening the valve more and causing the flow to increase. The effect is exponential.
This effect can usually be encountered where pressure differences are dramatic, at the cusp between water and air.
This is why your regulator so often free-flows as you dip it in and out of the water as you walk out into the sea, or plunge off the deck of a dive-boat.

ADDITIONAL CONTROLS
Many manufacturers get round this problem by including a pre-dive/dive switch on the regulator second stage. This positions a simple vane in the airflow to disrupt it and break up the venturi effect.
There is also an initial effort needed to crack open the valve. This can often be adjusted by turning a knob to tighten the spring tension on the second-stage valve.
In my view, if you want to inhale less air you simply draw on the valve more lightly.
However, these spring-tensioners can be used to make it slightly harder for the pressure-sensing diaphragm to lever the valve open, so can have the effect of correcting a badly set-up regulator second stage that might be leaking through its valve.
Many top-end regulators come with these two manual controls, the venturi plus/minus or pre-dive/dive switch, and a spring-tensioner to make breathing less effortless. People want added value with their purchases, and divers are no different. The question is, do you need them
Mares designed out the need to disrupt the venturi effect by using a patented bypass tube that feeds the air supply directly to the mouthpiece, rather than through the main chamber of the regulator. The company also eschewed using a method to increase inhalation resistance.
However, people still want knobs, and for the US market it has just introduced the Prestige 32 NTT, a regulator which has such a control.
It doesnt operate a movable vane, because the designers have already designed out the need for it. It simply restricts the amount by which the second-stage valve can be opened when in the minus setting.
Atomic regulators use a vane that is adjusted by a mechanical depth sensor, so that the user never needs to touch it.
Some other manufacturers, notably Coltrisub, have also now adopted this approach.
Of course, all regulators should be subject to a scheduled service, and its a good idea to take yours on a local dive to check that the technician has set it up properly before setting off on a dive trip abroad.

SURFACE FREE-FLOWS
So, if you have none of the additional controls, how do you avoid the annoying loss of gas that can occur just as you are about to start a dive
Avoid dipping the second stage and the octopus of your regulator in and out of the water if youre shore-diving. Also be aware that, should your regulator free-flow in temperate conditions, you simply need to increase the air pressure inside the body of the second stage by blocking the mouthpiece, or turning it front diaphragm up for a moment. There is never any need to smack it with a hand, or on a rock!

COLDWATER FREE-FLOWS
When diving in cold fresh water that might be only a few degrees above freezing, the cooling effect of depressurised air coming from your tank can cause ice to form around the mechanical parts of a regulator.
Some regulators are designed to counteract this, but be aware that water at less than 5°C can cause regulator icing, and that can include any fresh water outside the obvious winter period.
It is strange to think that what might feel like very cold water is, in fact, warming up the much colder air coming from the scuba cylinder, but regulators with plenty of metal in their design to conduct this small amount of heat to the air tend to be less prone to freezing.
You can help avoid freeze-ups by making sure that your cylinder is dry inside, and filled with dry air. Dont leave it anywhere that might chill it. Avoid heavy air-flows from your regulator, which includes breathing heavily from it.
Before diving, take test breaths when submerged in shallow water, rather than in the air, and be sure that your first stage is submerged at this time too.
Never press the purge button either above or below the surface.
Finally, restrict yourself to no-stop diving at depths from which you are confident that you could make a free ascent.