Here is a vid of me bending a six inch nail yesterday - they are easy.
I also have a high torque power twister which is much harder than a six inch nail.
Bending depends upon tendon and ligament strength with muscular strength irradiation as secondary. It also uses a lot of nervous energy and aggression.
I could bend a nail without grimacing.
How ever - bending is without a doubt the most painful form of strength training I have done in 26 years. Maybe because I am new to it. Maybe its neurological as the body is unaccustomed to the nerve irradiation or maybe just the load on the connective tissue.
Anyway it sometimes hurts like hell afterward for a couple of hours. I hope the pain goes as I get better and stronger.