I just had a thought maybe the sudden yank on the cord was the starter
spinning without the pawls going oput then all of a sudden the pawls
sprang out to engage - that would do it, but I don't know if that is
mechanically possible, I mean I can't remember how the pawls opened
outwards whether by cetrifugal force or was it just by tensioning the
pull cord they came out without any spinning/centrifugal force.


I can sort of answer this right now, looking at the broken starter on
my desk, it is clear how it works, it's not centrifugal force. There
is a steel plate which somehow stays still as the cord is pulled, as
the steel plate remains fixed and the rest of the pulley and pawls
begin to rotate, a couple of little protruding lugs on the steel plate
will engage with the pawls forcing them outwards, when they were
fully extended and engaged with the cup flanges the force is
sufficient to then turn the steel plate against it's resistance -
quite clever really.

Perhaps what was happening was that I pulled the cord the starter spun
around but the plate did not remain fixed so the pawls did not come
out. Then as I was pulling the cord out fast, perhaps the plate
suddenly fixed - the pawls came out and jammed onto the cup flanges.

So the answer would be to buy a new starter and make sure to engage
the pawls and the engine was engaged before putting real force onto
the starter cord.