The feature does exactly what it says on the tin: When a zone is underpopulated, the players in that zone will be given the option to form a group with players from a select pool of realms (presumably the server's battlegroup) with whom they can run about and quest as usual. In addition to treating underpopulation, the new feature will allow overpopulated zones -- such as new race starting zones -- to be split into multiple instances to prevent overcrowding. It will also allow players to group with their RealID friends just as they would for a cross-realm dungeon, so players who want to level up with friends on other realms should be quite pleased with the addition. Between this new feature and Guild Wars 2's guesting, the days of being separated from your cross-server friends might just be numbered. The full details on the new feature can be found at the official World of Warcraft site, so if you're wondering how this newfangled technology is going to work, just head on over and check out the handy FAQ.