Flex Plan is part of Weight Watchers. I'm not sure but now that I'm on the GI diet, I think it follows GI principles.
Phase One is from Rick Gallop's version of the GI diet. Not only does he promote eating low GI foods, but also those low in saturated fats. Food is green, yellow or red light and in Phase One (the weight loss phase), you should only eat green light food. Not as restrictive as it sounds.
I've been on this diet for a week and a half now and feel so much better now that I'm not eating refined carbs. I slid off the diet on the weekend with a couple G&T's and dessert and actually felt terrible and wanted to get back on it. Sorry, I'm in the beginning "evangelical" stage of this diet