That's pretty much how the good Dr. describes it. The reason to get off of phase 2 is so that you stop losing weight. Phase 3 is meant to keep your weight steady, without loss or significant gain. I think the 5 up/ 5 down philosophy sounds good. With phase three, you could probably take it month by month. Keep track of what you are eating, and if you go beyond that 5lb gain, you need to either decrease your food or increase your activity to burn the more calories you are taking in. If you keep losing, then, lucky you! You get to eat more.
I think making it to phase 3 is a huge accomplishment in itself
, and should give you the confidence that you'll never really have to worry about going back to phase two again.