It basically helps store a bit of energy, has no real effect on milage.
You think of it as an heavier flywheel,when it spins at speed it has a certain amount of energy stored in it. Because of its weight and rotational speed it kind of resists a change in speed so it helps maintain speed longer by releasing some of the energy in it, in other words it wants to keep spinning at the same speed. Hope this makes sense and explains the theory behind it and yes it work especially for crawling, at highway speed it will help a bit, but that is not really what it's intended for, it's more for climbing a hill or anything that try's to change engine RPM for a relatively short time period.