There's not a lot of other detail; Tesla simply said that "for Powerwall customers with backup and in selected regions,Tesla will automatically detect incoming storms and store energy." One customer noticed that according to the app, after the storm passes, "your system will return to its previous settings." The feature is timely in any case, coming just ahead of hurricane season.
Tesla also introduced a feature related to "Grid Services." It hasn't used that term before, but it's likely related to recent grid tests using Powerwalls in Vermont, Canada and elsewhere. In Nova Scotia, for instance, excess wind power is stored in Powerwalls located in private homes, then released around the grid when there's a deficit. Presumably, the new app update would tell users that they're generously loaning power to neighbors.