It's a little different for freedivers, IMHO this is not so much because of the inverted angle, but because you tend to descend so much more quickly. Here's an article from a freediving website that talks about how the technique works, how to do it, and most importantly how to practice it:
The Definitive Guide to the Frenzel Technique - With Emma Farrell - Go Freediving
In my experience, practicing the techniques can be done pretty well on the surface. You should be listening for the little click sound in both ears separately (for me, my left ear is always much harder to clear than my right, so I pay special attention to it).
Before you dive down, you can pre-pressurize your ears at the surface, and then immediately dive down before they equalize back to 1 atm.
As the gap between ambient pressure and your middle ear grows, it becomes more difficult for the eustachian tube valves to open. So, once your head is underwater, clear immediately, and put effort into clearing every body-length or so, for the first several body-lengths. As the depth increases, you can decrease your clearing frequency.