I one does a Google search for "bearing preloading DF" it turns out that the VERY FIRST search result that came up was:
http://www.bardenbearings.com/PDF%20CD/PB11.PDF
It can't be explained any more simply nor anymore concisely.
Theoretically, a ball screw an be back driven - BUT depending on the ball pitch (as in turns per inch), it may or may not be easy to do so.
Torque is expressed in newton/meters whereas torque in USA is usually expressed in oz-in or ft-lbs. The use of simply math should enable you to convert newtons to oz or lb force and meters/mm to feet or inches.
A 20 turn per inch screw will be harder to back drive than a 10 turn/inch which will be harder than a 5 and so on. Factor in the gear reduction back to the motor and the ease changes even more. A 1:2 motor/screw drive will be easier to back drive than a 1:1 which will be easier to back drive thatn a 2:1