Bottlecap question - eviltoast

Hidyho.

A few months ago I was watching a YouTube video of a woman bottling her kombucha. She had bottlecaps that would bulge when the carbonation was just right.

I looked for those tonight via searching on Google but couldn’t really find the right thing.

Does anyone know what I’m talking about?

This would be perfect for my kombucha and water kefir tester bottles.