Call forking support means that developers can now forward calls from VoxImplant to VoxImplant application user that is connected to the cloud from different endpoints at the same time using the same login credentials. For example, let's assume you have a SIP phone registered on the platform and Web or Mobile SDK connected and authorized on the platform, if you use callUser function in your VoxEngine scenario all the endpoints will ring at the same time. The first endpoint that will answer the call will be connected, while all other endpoints will stop ringing after that. Most likely you've seen this behaviour before when somebody called your Skype that was running on your PC and mobile at the same time.
This is very convenient for the case when your users are working with your app on different devices - you let them choose the device that will work better at the moment.