You set both apps to sync, and then they do, both ways hopefully. I continue to log food on MFP, since I have been using for a while, and exercise on the UP. I also link the Fitstar and Six Pack apps, although the latter syncs to MFP but not to UP. Outside that, I set the timer on the bracelet when I am doing any additional exercise that's more than just walking: eg exercise bike, or when I walk up the steep hill to my house. I set the latter as a hike, in the zone, although judging the level for a particular exrecise is a very subjective thing.
Like many, I have had problems with my bracelet, after less than two months. A new one is on its way. Anecdotal evidence is that they fail a lot, but of course we only hear about the failures and not the successes. And Jawbone is not likely to be revealing the actual percentage failure rate. My main concern is that I believe the warranty only covers the year from initial purchase, and there have been tales of multiple failures in the course of that year. Once it expires, Jawbone presumably won't accept responsibility any more.