To learn (and train) frenzel, you could try this exercise in the pool (I got it from my instructor few weeks ago). You need to know the theory on how frenzel works before:
Dive to the bottom on exhale, head down (this is important, as it requires you to push the air in the opposite direction it would go naturally). For the first tries a mild exhale is more than enough. When reaching the bottom (don't forget to equalize on the way), try to fill your mouth with air, and then try to push the air out through your nose (with a mask on). To do this you have to use the tongue, it just won't work by pushing the air from your lungs. You have to see bubbles coming out of your mask. Sounds easy, but you will see the first time you try, that only way to do it is relax and concentrate on what you do with the muscles in your mouth.
And don't forget to watch out for the position of your head (no looking up
), as you can easily damage your trachea on an exhale dive even in the pool. And don't spend too much time down there, just a few tries and head back to the surface. Having someone spotting you is necessary. Best is to do it in pair, one is down, the other one breathes up and watches.
