Wood Harvester Automation adds some automation to all wood harvester:
When the machine is on and ready to work, it will show a new hud. It will show three pre-set length and one function for auto, full-auto, manual and off operations.
It also becomes possible to cut any type of tree, when you are near a tree ready to be cut and when a tree is grabbed, you can see projected informations about type and size.
Quick guide: activate mouse pointer with left ctrl key and go over the icons on hud, set desidered pre-set length with mouse wheel, activate with click, try auto or manual operations with click on right icons or hired helper key (default H).

...the best way to understand everything is to try! cut! cut!! cut!!! the default help in game contain a page dedicated to all Wood Harvester Automation function and behaviour.