Hello,
On our units, the DI bypass serves two purposes. The first is to bypass the DI and get just RO water (which we have another vavle for that on the line normally - like our Typhoon III). The second is to aid in the flushing of the membrane. This is the top part of the assembly. It goes to the drain line and is installed after the flow restrictor.
I hope this clears some of the confusion up.
Regards,
Air, Water & Ice staff