Hi Adrian,
I am now also back at learning the Frenzel properly. I found out that I already do it but mixed up with Valsalva somehow.
I found this from the Finland guys:
http://www.freedivingfinland.net/nuk...icle&artid=164
And then I also heard that doing it while being horizontally helps - but I manage that okay. If I am not going really straight do I can equalize up to about 20 metres.
But not with the head being really down - that does not even seem to work on dry land.