Whenever we submerge ourselves in water, the weight of the water above us puts pressure on the eardrum, and the deeper we go, the greater the pressure. In order to dive without injury we have to equalize (or 'EQ') the difference in pressure between the water we're diving in, and the airspace inside our ears. If we do not equalize this pressure we will feel pain in our ears, and--if we continue to dive deeper--we could rupture our eardrum. We equalize our ears by pushing air into the eustachian tube from the back of the nasal cavity.
There are a variety of techniques for equalizing the ears, but for freediving we emphasize the Frenzel technique. When we Frenzel, we are using muscles in the jaw and throat to push air into the eustachian tube. If you have signed up for a freediving class, take the time to make sure you can Frenzel. And if you are one of the lucky ones who can just 'hands free' equalize--particularly if much of your handsfree experience is with a scuba tank--you still will want be familiar with the Frenzel since it may be necessary for you when you get particularly deep, or if your ears get 'sticky' during the class.
While the skill doesn't take a huge amount of time to master, it can be very difficult to learn and apply in a single weekend class.
But, here's the good news: You may already be Frenzelling without knowing it.
Here is how to test: Take one hand, relax your body, laying down if necessary, and poke your stomach just under your ribcage (this is your diaphragm). It should feel soft, with no tension in the muscles there. Now, with your other hand, pinch your nose, and pop your ears. Keep poking your diaphragm with the other hand. It should remain soft and relaxed. If you are equalizing correctly, you should be able to pop your ears without tensing any of the muscles in your stomach or ribcage, even for a moment. If that's the case, you're doing it right and you are already Frenzeling!
If you are unsure even how to 'pop' your ears, or have difficulty keeping your diaphragm relaxed when you do so, I recommend you do a little homework.
We'll start with just a simple video. The first one on the list is from Aharon Solomons, is a British freediving coach. A lot of my own training philosophy and methods are a bit different than his, but he won't advise anything unsafe, and this video is very detailed: https://www.youtube.com/watch?v=BeL5dI5Hkg8
If you like, here is another one, a little more technical, by another instructor named Adam Stern: https://www.youtube.com/watch?v=Mo07gZR741M
You'll want to budget about 40 minutes to watch one of the videos. They are shorter than that, but you'll want to pause, rewind, and probably use a mirror as you go. Once you can get a dry Frenzel working, you're on your way. Over the next few days, just keep practicing 5-10 minutes a day. It isn't necessary to spend hours on it, and in fact that may be counter-productive. It also is not necessary to practice this skill in the water, there will be plenty of time to do that in the class. Just do a few minutes in the morning, a few minutes in the afternoon, and a few minutes at night. We're just trying to make it something you can do nearly by reflex, with very little effort, almost like blinking. Ideally, you should do 200 gentle Frenzels in a day. This may sound like a lot, but when you're Frenzeling properly, it can be done very, very quickly--2 or 3 times per second even.
Most people can master the Frenzel this way within a week or two.
Here are some more articles on EQ in case you want to delve deeper:
Information from Hawaiian diver and instructor Kurt Chambers: https://www.facebook.com/notes/kurt-chambers/equalization-by-kurt-chambers/398607186955804
Information on treating EQ problems resulting from ears which have been stressed by diving, from physician and PFI Instructor Kerry Hollowell: https://www.performancefreediving.com/hears-to-our-ears
For those who have mastered standard Frenzel and are looking towards the next step, you'll want to work on developing conscious independent control of the soft palate and epiglottis: