Zaq,
There is a difference between configuration sync and connection sync (called mirroring).
Configuration sync is configured in so called Device Groups and is only applicable for configuration changes. You can configure auto-sync. A change on the Active member is synced right away to the Stanby member. When you disable auto-sync, you manually have to sync the configuration to the Standby member.
Connection sync (mirroring) is configured on the virtual server by selecting the Advanced pull down menu in the virtual server configuration page. Keep in mind not all connections can be synced (using client en server SSL profiles for example) and it may have impact on the performance.
Martijn