On my first try of box-breathing, after roughly 7 minutes of doing the practice I burst into a tearful laughter and couldn't stop. It was an amazing feeling, now I would call it lung-orgasm, but after 1-2 minutes I kinda got scared about it and managed to stop in the end.
I tried to reproduce it after, but it never happened again. Maybe it requires some built-up tension and that day I managed to release it.
Sounds similar to a 'jhana' state in Buddhist meditation. My understanding is that there are layers/steps of meditation practice and the higher level ones may allow such states; people can sometimes find themselves temporarilly jumping up a few layers and getting such experiences.
After some articles recently about this on AstralCodexTen I'm reading 'The Mind Illuminated" to try and decide for myself if such "unfalsifiable internal states" exist.
I tried to reproduce it after, but it never happened again. Maybe it requires some built-up tension and that day I managed to release it.