I learned the hard way when I was younger, not that I am real old now 38, but I grew up with antique cars, trucks and tractors with hand cranks. However, I never broke my arm, but it really hurt for several days. Most of my hand-cranking skill came from the ever so famous "Model T" Ford" and my "Model A" Ford. I have had a couple of old tractors that had either hand cranks or places on the flywheel for starting the engines.
Most of the old cars and trucks had a "spark control" lever. what this was is a lever on up on the steering wheel column and many people would forget to push the lever back up which in most cases was the "retarded" position and they would try to hand crank the engine and this was when they really like to kick back and break your arm. I can't think of or know if there was ever any tractors with a spark advance lever or not, none of mine did.
As like John *.?-!.* cub owner said a well tuned engine will "ALMOST" (and that's in quotes) never kick back. I have also learned that if the engine is tuned poperly, all you have to do is get the enigne on a compression stroke and just pull up on the hand crank and it will start.
I know more people who have gotton hurt by hand cranking an engine if they crank full revolutions. Just like Donny said, keep that thumb on the side of your crank, don't wrap it around.
On our "Model A and T's I will get it on compression stroke with the choke on and then if it doesn't start, that usually leaves the carb. spewing out gas and I will take off the choke and give it another half turn and it's running.
Now, I haven't used the crank on my Cub yet, but it's so easy starting, I doubt if there will be any problems. I just have to do a little work on the bolster, it froze up once and cracked everything either some brazing weld got inside the hole or the hole itself is messed up because of the freezing up, so I have to run a reamer in the crank hole and hopfully that will fix the problem.
You know stop to think of it, I have actually gotton more hurt kick starting the old Harleys that I use to own than I have ever using a hand crank on an old car or tractor engine. My knee still goes out on me because of them dang old Harleys I once had. Thank God for electric start on my newer one.
Happy and SAFE hand cranking,