Concussion bellows are small spring loaded bellows that I think are usually attached to the wind trunks in an organ. Mine attach to the back of the windchest. The purpose of concussion bellows is to prevent spikes or drops in pressure as the demand for wind changes. It's the equivalent of adding a capacitor to an electrical system.

Here are the concussion bellows before being leathered:

The leathering process:

Leathering complete:

I made the spring so that when the bellows are inflated, the floating plate sits about halfway in it's travel:

Mounted to the back of the windchest: