Residual Volume

Understanding residual volume and the mouthfill technique, and subsequently breaking through the depth at which you really need to understand these subjects, is sort of a dry and technical matter.  Unsure how to tackle the subject, I decided to rely on a fundamental technique utilized in educational institutions across America: plagiarism.  Joking aside, it's not actually plagiarism, I give full credit to whoever wrote this article:
The mouthfill- how to equalize deep in freediving
Also check out:
The Frenzel Technique, Step by Step

To practice these techniques, I'm using FRC dives and variable weight (as can be seen in Deeper Perspectives- Episode 6).  It really only makes sense to perfect equalization this way, rather than trying to head straight for the abyss.  Can you imagine trying to learn to drive, grinding gears, and riding your brakes  on the Autobahn?  Of course you wouldn't do that; you'd find an empty parking lot, a back road, or some other controlled environment.
Variable weight obviously allows you to concentrate on your equalization technique without the bother of all that kicking and oxygen consuming activity.  FRC allows for short and shallow dives that can be repeated many times in a session and easily monitored by a spotter.  I still don't precisely know what volume of air I'm working with on an FRC dive.  I think it's something like 40% (2/5) of my total lung volume.  By going to only 20 meters, I'm able to reduce that volume to 1/3 (http://www.ehow.com/how_7436_understand-underwater-pressure.html).  1/3 x 2/5 = 2/15.  This would be the same lung volume experienced at 7.5 atmospheres, or 65 meters.  In other words, you can experience the same pressure and equalization demands of a 200+ foot dive in less than 70 feet.  Of course, this also means you are susceptible to injuries normally reserved for deep depths, most notably lung squeeze.  So bear that in mind; this isn't professional instruction, I'm just some chump with a blogger account.