It only works with the Pro Edition, if you have the Home Edition like me it's not available.
Directly from from avast help within the program....
Push iAVS. It is a special kind of update. Sometimes, it might be very important to perform the update as soon as possible (e.g. when a new virus starts to spread heavily). With push iAVS, it is possible to start the update remotely, on the request of ALWIL Software specialists. The request is done by a special e-mail message. avast!, checking the incoming e-mails, will recognize the message and automatically starts the update.
Enable. Enables the push iAVS function
Add information about performed action into e-mail. If you select this option, avast! will add a note to the message saying if and how the update was performed.
Registration. The push iAVS service must be activated by registration. After clicking the link, an on-line form will open in your browser; here, it is necessary to enter the e-mail address messages to start iAVS will be sent to.