As you may already know, you can take advantage of redundancy with your Azure storage accounts.
If you use geo-redundant storage (GRS) – either GRS or RA-GRS (read access), you can now control when the to failover if the primary location is unavailable.
As this is a preview, this currently only available in US-West 2 and US-West Central regions.
If you want to control the failover, you can do it either from the Azure portal, Azure PowerShell or Azure Cli.
You will first need to register the provider to your subscription either by using Azure PowerShell
Connect-AzureRmAccount -SubscriptionId <subscription-id>
Register-AzureRmProviderFeature -FeatureName CustomerControlledFailover -ProviderNamespace Microsoft.Storage