Thank you for asking. I've been building for a Market of One --- i.e. myself (heart failuer). Research shows breath holds are not recommended for my condition. Having said that, I realize this is not a universal guideline. I'm considering adding other modes besides heart failure-friendly ones. What are your use cases? Would you like to submit a feature request?

The readme says "This app deliberately does not support breath retention, rapid breathing, or any pattern not grounded in the slow-breathing clinical literature." Also links to relevant literature.