[glennv]

p.s.
You can still do a very good frentzel with pressure on your cheecks and lips.
Do not confuse using your tongue with doing a correct frentzel.
Like Eric Fattah described (and what i personnaly find easier to start with) is use your cheaks to pump the air from your mount to your ears (so not from the lung of course).
Using the tongue you can create higher pressures because of the smaller volume to compress, but it is both the frentzel. (eg neutral eppiglotus and closed throat).
I found out after long experimenting on what the hell i did wrong, that to forget about the tongue (for the most part of the dive at least) and just use small contractions (or a very small constant pressure) of the cheaks on a filled mounth , suddenly everything fell into place. Probably because when you focus on tensing your cheecks , you can relax the back of your throat and it is easier to control/relax the eppiglottus. On the more difficult last part of the dive I will then focus on the tongue to generate some extra pressure on the last bit of air. If I start from the beginning with the tongue I loose it halfway (start blowing air against a blocked eppiglotus, or air escapes back in my lung).
Will get easier by time for sure.
I (as do lots of freedivers) still train almost every day to learn how to control the epiglotttus better and better.