Back to the OP.
Do any of you, er, more mature, ladies & gents remember the chap on the headset delivering a "positive N1" remark during start?
I still do it occassionally when starting engines in to a tail wind. The big fan engines will light off and N2/N3 rotors will happily accelerate while the N1 is still going the wrong way. CF6 seems to be the most common. Autostart doesn't seem to worry about negative N1, however, a smaller fan engine such as the V2500, may be more susceptible hence the FCOM requirement.
Oh, and by the way, the fan drives the rotor/LPT when windmilling in either direction.
As BOAC states, the old trick of deploying the reverser is to block the airflow through the cold stream duct therefore removing the driving force on the fan.
My company's policy for engineers is to max motor the engines during start due to an embarrasing incident on a 747 some years ago when 3 donks got trashed during engines tests.