This page allows you to enable and disable the VNS. I would recommend turning on the first two options and leaving the 3rd option disabled until you become familiar with the system. Please understand if you enable backward flying during AT the copter could run into an object since it can not see going backwards.
Remember you can still control distance from object with elevator no matter if this option is on or off while in Active Track (AT) mode.
VNS Example RTH: