I think he's filling up with water and eating less.
Same idea as the slice of 100% whole wheat bread 30 minutes before eating, and apple 30 minutes before every meal, or taking fiber suppliments with water 30 minutes before every meal...fill up on something with
low calories and don't eat as much.
It works, but you've got to be careful with something like that to make sure you are actually getting the right nutrients you need...it borders on a very
low calorie diet if you don't watch your intake closely.