You have to rig the breathing first, you don't need this to simulate breathing as it can be done using Session only.
Viewing post in Puppetstring VTuber Tracking comments
thanks for a quick reply, i didn't know at the time how to write an expression as opposed to a ratio in the tracking window or even what those words meant.
For those finding this thread later, how to breathe is on this page of the documentation (was hard to find as the homepage just says WIP) https://github.com/Inochi2D/inochi-session/wiki/Expressions