The lungs' residual volume is the volume of air left in the lungs after a full exhalation. For a regular person, the residual volume is 1/4th to 1/5th of the total lungs capacity. When we freedive in open water, as we go down, we need to equalize our ears, and to do so, we bring regularly air from our lungs that we pressurize in general using the Frenzel maneuver. When going down the pressure increases and therefore the volume of our lungs decreases proportionally (Boyle’s law).

That means that when we reach 4 to 5 bars of pressure (30m to 40m), we reach our residual volume. Since the residual volume is the volume of air left in the lungs after a full exhalation, we cannot bring air anymore from our lungs to our upper airways in a natural way. We cannot equalize anymore. 
With training, we can reduce our residual volume. Some top athletes have a residual volume close to 1/10th of their total lungs capacity. Still, at one point, they reach the depth of their residual volume where they cannot bring naturally air from their lungs to equalize.

What Is The Mouthfill?

To overcome this limitation, the most used equalization technique used at depth is the mouthfill. The principle is very simple. Before reaching our residual volume, we bring enough air from our lungs to completely fill our mouth (cheeks fully inflated), and then we will use the air stored in the mouth to create the pressure necessary to equalize at depth. The principle may be simple, but the use of mouthfill is actually technical and needs constant refining for most freedivers in order to achieve greater depths. 

If mouthfill can be tricky to use, it has to do with all the anatomical parts involved of which we have very little awareness. To perform the mouthfill in an efficient way, we need to have good control of our tongue (and not only the front part … yes the tongue is a big muscle and most of the tongue is in the back of the mouth), of our soft pallet (the soft pallet can be trained and controlled precisely) and our glottis (had you actually thought about your glottis before taking a freediving course?). 
I will not go into more details about the technique of the mouthfill but to summarize, the tricky part is that if your glottis opens even a little, some air is sucked back into your lungs. Plus the air that is in your mouth and that you use to create the pressure to equalize is shrinking as you go down so creating the pressure becomes more and more difficult and requires rearranging the air in your mouth while keeping your glottis closed. 
It is now easy to picture why you want to avoid any physical distraction in the area where the parts involved are located. I would say particularly, front neck, chin, and throat. 

How Can The Alchemy Neck Weight Help Your Mouthfill

My past experiences with artisanal neck weights is that they would always one way or another put some stress on one of these parts and disturb my mouthfill. With the new Alchemy neck weight, there are several lengths, so you can choose the right one so that the end comes and rest lightly on your jaw. You can also shape it and reshape it as many times as you want. The high-quality manufactured led allows it without being afraid of breaking it. It is easy to adjust the shape perfectly so that you do not get any pressure or distraction on the throat or chin. 
Finally, the silicone material makes the Alchemy neck weight stick to the smooth skin wetsuits. The neck weight is not moving around at all during the dive. One less distraction. As I am favoring smooth skin wetsuits, it is a perfect fit for me.

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