The rubber patch is sealed all around, Kevin, and I’m sure no air escapes. I didn’t think to try blowing while able to view the patch, but I’m sure it expands and contracts but not with any balloon effect. I think your suggestion about it’s serving to reduce the shock on the reeds makes sense. When I felt a bit of give and take that made me suspect something like this, I also could hear a slight flapping sound in both directions, like a diaphragm maybe? The material is thin, firmer than the image you may be having, flexible enough to become concave or convex but not to inflate. It’s clearly meant to go out and in with the air pressure, so that it buffers the air flow. I’m sure there are other instruments and devices that involve similar design and that there must be better terminology than what I’m using. I would be afraid that blowing as hard as I could into one of my favorite melodicas might cause damage.
It occurred to me that someone who had the inclination and reason for doing so could probably install a similar device on any melodica. Maybe you and I could go into the aftermarket melodica baffle business. Between us we could come up with a list of benefits: protects your reeds, stabilizes tuning, facilitates circular breathing, creates jobs, cures the common cold.