Inside the magic band are 3 radios: 13.56 MHz near field RFID (what's in your credit card's swipe to pay), 915 MHz passive UHF RFID (what's in your timing bib if you run a 5k race or what Walmart uses for inventory tracking), and a 2.4 GHz active radio.
The following are my conjectures on how it works. I do not work for Disney or any contractor, but I do work in the field of RFID.
The 13.56 MHz near field RFID is used for payment and entry to the park, and any other time Disney has you hold your band up to something for a transaction. It's near field, so the range is only a few centimeters, which is good for payments and when you want to ensure only one band is being interfaced with. This RFID consumes no battery power from within the band.
The 915 MHz UHF RFID is used for positioning with shorter ranges but near instantaneously. For example, when you sit at a table, my guess is they are using this UHF RFID to find your location (an antenna at each table gives very good indication of who's at that table). Generally range on UHF RFID is a few meters, plenty to detect who's walking up to the dining location if they funnel you through a specific path (called portals in the industry). This is also likely the technology used to detect which car/boat you are in when they take your pictures on the rides. UHF RFID can acquire hundreds of tags per second and consumes no battery power from within the band, which is perfect for quickly detecting who's in range of a given antenna if they are only a few meters away.
Finally, the 2.4 GHz radio. It's active so does consume battery power and the battery inside the bands are tiny, so it cannot be transmitting that often or the battery life would suck. Hence, I believe that the near field interactions reset a timer which runs for a few days to a week to enable the 2.4 GHz radio. Likely this radio transmits every minute or so. Disney do have receivers around the park for this 2.4 GHz radio system and I believe they only use it for detecting rough location of all visitors which is perfect to determine the lengths of lines at rides or to find where a lost kid is in the parks. Once the timer expires (if you've not interacted with any near field readers in a few days) then the 2.4 GHz radio stops until the next interaction. This is likely the only way to get such good battery life and how they can ship the thing to you weeks before you arrive.
EDIT to add: That 2.4 GHz radio probably has a range of somewhere around 30 to 50 meters (possibly more). Some of their receiver stations are in the lights around the park (some with wired Ethernet others with wifi or other backhaul RF networking), others are little black boxes with 100BaseFX (fiber Ethernet for long runs like 1km). This is perfect for knowing crowd density in an area and estimating line lengths for outdoor areas (the UHF RFID could also be used for line length estimation in controlled or indoor areas). Probably they also have some 2.4 GHz receivers installed in the parade floats and vending carts (along with GPS to know where those items are) to collect data for location tracking.
All in all, Disney are executing the wearables and RFID systems exactly as they are best envisioned. It's awesome! (if a bit scary)