Crank only on the pull, never on the down.
Even without your thumb being wrapped around the crank, you could still break an arm.
After witnessing an F-20 backfire on start ... I did not need to be told twice to not rev it around.

All this talk about how to crank an old Farmall is also telling how darned OLD some of us are.
I worked for a Farmall dealer back in the mid-fifties.I got paid $54 for 54 hours of hard work.
I cranked many of those old F models .Never got hurt bad. Had a good teacher.
The H's and the M's were/ are a nice tractor.