If the Enable reverse Alerting checkbox is checked, Alerting is not only performed when an event occurs but also when it clears. If email Alerting is ON, an email is sent when a device goes down AND when it comes up again! If a value exceeds a certain threshold and a script is launched, the same script is launched again when the value drops below the threshold etc..
A default message is e-mailed or spoken if the Enable reverse Alerting checkbox is checked and a pending alert is acknowledged by the network-manager!