Once you've got the actuating arm locating bolt out and the end cap off you can loosen the main outer sleeve. You might be able to pull it off – if not carefully persuade it with a screw driver blade (eeek be careful) – better still tap it with a rubber hammer!
You can now withdraw the mechanism that engages with the flywheel (the drive assembly) away from the nice aluminum alloy casting which also holds the solenoid.
I do like these castings! These Bosch starter motors are very well made – which is why it is a shame to junk them.
Take out the actuating arm and yoke.
You'll also find a rubber cover and a plate here.
You can now pull out the armature assembly away from the outer casing.
Inside the main casing you'll see four field coils.
To see how an electric motor works have a look here:-
http://en.wikipedia.org/wiki/Starter_motor
That's as far as I went – you can pull the drive assembly away from the armature – but mine was nice and clean – freely moving – so I left well alone. To see more on how to do this have a look in the pdf file – link posted above.