Thanks for the link.
The video is really interesting.

From the video and the underwritten notes I understand that the Frankel Appliances

1) are used on very young patients and not on adults
No, also for teenth and in exceptional cases for adults (not for active treatment, but for retention). I myself got the first function regulator as schoolboy at 14 or 15 years. The second one I had to wear as adult during second treatment as retention appliance for training of the face muscels.

2) are loose in mouth, not they do not clasp on the upper jaw, so thaqt when you open mouth fro example for speaking they move inside the mouth
Yes, it is not fixed at the teeth.

3) are worn FULL TIME for a long time
Yes, the appliance has to be worn full time (24h per day / 7 days per week) and should be removed only for eating and sport activities.

They are BIG and fill the mouth, also because they are somehow "elastic"
The large plastic parts are between teeth and cheeks, so there are only wires and little plastic parts in connection with the tongue.

It definetely looks like one of the most impressive appliances ever! Maybe more or lesso like a twinblock!

It is pretty noticeable also with your lips shut, because they give youa sort of "squirrel loook" with ceeks full.

Despite the fact that they are inappropriate on adults, I'm curious to find how it feels inside my mouth!

I really cannot imagine how can you speak with it inside.
Speaking is not a large problem with the appliance inside, but you have to learn to speak without big movements of the jaws. The teeth have to be nearly closed during speaking. Therefore some training is required, but after this there is no problem.