Fast Mode shortens the sync interval to five seconds when messages are ready to upload, but have not been uploaded in the last regular sync cycle. Fast Mode does not get enabled as soon as an update is saved on the mobile device. The normal sync cycle runs at the set interval. During the next sync cycle, the sync engine will try to upload a batch of messages. If all queued messages get sent to the mobile service in the sync cycle, the sync interval remains the same. However, if there are still messages queued that have NOT been sent to the server, then Fast Mode is enabled. Once all messages have been sent, Fast Mode is disabled.
This feature is designed to speed the uploading of large numbers of messages, especially if the user has been working offline for awhile and returns to an area of connectivity. The best way to test the feature is to put the device in airplane mode (or pause sync) and make dozens of updates so that there are dozens of messages in the queue. Then go to the sync screen and watch how fast mode is enabled after the first sync interval after returning connectivity to the device.