Seems to be confusion in this thread, and broader in the hobby as well. A FLUSH VALVE, and a DI BYPASS are two very different things and they are used for very different purposes.
A FLUSH VALVE is either a manual or an automatic valve on the concentrate (aka "waste water) tube that temporarily allows the user to bypass the flow restrictor. It allows water to rush through the membrane and membrane housing to remove debris. This is done in an attempt to increase the usable life span of the membrane.
On the other hand, a DI BYPASS is on the permeate (aka RO water) tube and it allows the user to either direct the permeate to the drain, or direct the permeate to the DI stage. This is done to minimize the DI capacity exhausted by TDS creep water - the relatively high TDS water produced by the RO membrane when a system kicks on.
Russ