A flushometer is a metal water-diverter that uses an inline handle to flush tankless toilets or urinals.
It is the narrowness of this passage that meters the flow by slowing repressurizing of the pressure chamber after the action of a flush.
A groove in this cylinder allows water from the main supply to flow through when it is in a mid position.
The topmost pressure chamber slowly refills through its narrow passageway, pushing the valve cylinder back down gradually.
[1] A flushometer is usually installed in a commercial setting with the exception of some older apartments in large cities,[2] as it provides a high-pressure and better-performing wash and flush than a normal gravity toilet.