Equalization Training Device (EqT) for Freediving

Just connect the EqT to a balloon and start your exercises for Frenzel and Mouthfill. There are lots of tutorials out there of how to practice this. Just search for e.g. "frenzel balloon freediving" and you'll find various videos and other explanations.

Some credits: MJN031 has designed one of those devices and shared it here on Thingiverse (https://www.thingiverse.com/thing:2221460). This definitely does the job as well and works perfectly fine, however: I have printed one, and I personally