Flushing is pretty much a money grabber.
But I do it lots. You have to buy a "Flush kit" for your particular brand and model. You transfer your flow regulator thingy (they have a name...I just can't think of it...) into the middle of the flush kit. When the knob is one direction it bypasses the regulator and flushes the membrane. When the knob is the other way the regulator is in effect and you get RO water.
The Flush kits are usually like $15.