Mailing the story:

Idle Scans are performed against a Target System without any traffic occurring between the Source and Target Systems. The Idle Scan is accomplished by using a third system to pass requests between the Source and Target. The third system is referred to as a “Zombie” System. The “Zombie” System will receive requests from the Source System and send them to the Target System. Responses are passed to the Source System. The Idle Scan is sometimes referred to as a Proxy Scan since the scan is performed through a third-party system.