An alternate second stage (also called an alternate air source, buddy regulator, or octopus) does exactly the same thing as a primary second stage. The difference between a primary second stage and an alternate second stage is that the alternate second stage is not intended to be used except in the case of an out-of-air emergency. A diver with an alternate second stage can allow an out-of-air diver to breath from his tank without putting himself at risk.
1. Mouthpiece
The mouthpiece is the part of the regulator second stage that a diver bites down on. Alternate second stage mouthpieces should be a standard size to fit any diver's mouth; not a custom mouthpiece. The idea is that any diver should be able to use the mouthpiece in an emergency.
2. Low Pressure Hose
A low pressure hoses (LP hoses) transport air from a regulator first stage to its second stages. An alternate second stage's LP hose is usually longer than the LP hose attached to the primary second stage. This extra length makes it easy for an out-of-air to use an alternate second stage attached to a tank he is not wearing. The LP Hose attached to an alternate second stage is frequently a bright color, such as yellow, to make it easy to see.
3. Purge Button
The purge button on the alternate second stage has the same function as a purge button on the primary second stage -- to remove water that has entered the second stage. Alternate second stage purge buttons are usually brightly colored, this one is neon yellow. The bright color makes it easy for an out-of-air diver to locate the alternate second stage in an emergency. In general, the alternate second stage should be attached to the Buoyancy Compensator (BC) or diver somewhere between the bottom of the diver's chin and the lower corners of his rib cage.
4. Ease of Breathing Adjustment
Just like the ease of breathing adjustment on a primary second stage, the ease of breathing adjustment on an alternate second stage can be used to increase or decrease breathing resistance during a dive. If an ease of breathing adjustment is present, divers should adjust it so that the breathing resistance of the alternate second stage is increased. They should also turn any pre-dive/ dive adjustment to "pre-dive". The regulator will still work if needed, but this adjustment will ensure that the alternate will not free-flow during the dive.
