не могу понять - она передвигается по thrust log или устанавливается в передней/задней его части?

Тем более там же объясняется, каким способом перемещается - using a hydraulically powered Nut Driver
(кстати, он, очевидно, потому и называется Nut Driver, что перемещает Thrust Nut)

forward и backward - куда , а не где
вперед и назад, а не спереди и сзади

И в примере Liza777 то же самое
facing forward/backward - смотрит (куда?) - вперед/назад

The Scissor Group is mounted on the end of the Ram Cylinder, and when the Scissors are open, the Thrust Nut and the Thrust Log can move through the Ram Cylinder bore. When the Scissors are closed, the Thrust Nut (and therefore the Thrust Log) cannot retract past the Scissors or the Ram. The Slide Cylinder could move the Tailstock forward: not backward. This obstruction is used to provide a stiff force-path from the Ram Cylinder through the Scissors, through the Thrust Nut, through the Thrust Log, through the Tailstock Fixture, through the X-Y-Tooling Plate, and the Fixture Tooling, into the parts being welded and opposed by the Spindle on the same centerline. With the Nut and Scissors positioned properly and closed, the Ram’s 12 inch stroke can be used to rapidly move the Tailstock towards the Spindle and apply the forge load required for welding. After welding, the Ram can be retracted, the Scissors Opened, and the Tailstock can be retracted using the Slide Cylinder to the load/unload position