This is probably way off the mark, but the first thing that came to mind was a shock absorber. Sometimes opening and shutting valves can create a kickback. In copper piping systems, the plumber traditionally installs a vertical dead end leg to accumulate air as a compressible medium to stop "water hammer". Just as likely, the air can be siphoned off or reabsorbed in the water flow. Ideally, this leg would be accessible and have a removable cap to "recharge" the chamber, but few are.