Explanation/Reference:
APT Blocker is intended to stop malware and zero-day threats that are trying to invade an organization's network.
APT Blocker uses a next-gen sandbox to get detailed views into the execution of a malware program. After first running through other security services, files are fingerprinted and checked against an existing database - first on the appliance and then in the cloud. If the file has never been seen before, it is analyzed using the system emulator, which monitors the execution of all instructions. It can spot the evasion techniques that other sandboxes miss.
Reference: http://www.watchguard.com/wgrd-products/security-modules/apt-blocker