There is no user intervention required to enable this feature. The Google Analytics intelligence engine goes through your historical data so that you can go back and review automatic alerts from the past.
As you can see in the above example, Google Analytics has flagged an event where Pageviews by visitors coming from to our website from New York has gone up over 60% in November.
You can also sort all the alerts based on either dimensions or metrics by simply clicking on the Metric/Dimension tab. In the following screenshot you can see that location.
You can manage alert sensitivity by simply moving the Automatic Alert Importance slider between high and low settings. High Sensitivity settings will allow you to see all the alerts that Google Analytics created and the low sensitivity settings will display only the most significant alerts.