It's hard to tell from the picture, but I believe you are correct about the left valve being the DI bypass. It is on mine anyway and the Flush valve is the only one that has a black hose.
Before you make your water, turn on the flush valve for about 30 seconds. This will bypass the membrane and get the old water out of your system without causing extra wear of the membrane. You can tell if you turn the flush valve because the water will run out fast through the waste line. Leave the valve on the good water line turned off.
After you flush the membrane, turn off the flush valve and turn on your DI bypass. Open the valve on the good water line and you will get water out as usual, but it will only be RO water. I let this run for a few minutes and then I turn off the DI bypass and make my water.
Hope that helps.