I had mine done two weeks agao, they had me use a fleet (flush) from both ends. I had to flush the unused side of my stoma as well as a flush from the rear. It doesn't make any sense to drink that stuff and just fill up your bag, unless that's what your doctor wants? After they did my colonoscopy to make sure I heeled enough for the reversal, at the hospital on the day of my reversal they used a rubber tube and stuffed it in my stoma (the side connected to my colon) and flush with about
16 oz of fluid. This was a nightmare as I cramped immediately and broke out into sweats. I did pass the fluid within 15 minutes, thank god, then it was off to surgery. Hope this helps, I would double check with your doctor!