I mistakenly said back flush, like in a filter system.
In an RO system, flush is the concept.
Your waste line has some sort of flow restricter in place, to keep the pressure up on the at the face of the RO membrane. A flush kit or system, by passes that restriction and allows water to leave the membrane housing at the same flow rate it's coming in. That washes out some of the minerals, and loose carbon from filter changes. For normal operation, the flush valve is closed. Parts are two Y fittings, a valve, and some spare tubing.