I am a Sox fan and watch most of their games. Hannigan leaves his throwing hand out exposed quite often. That is the third time in the past few weeks that it has been hit. About a week ago, I predicted to my wife that he was going to get injured. This time I hate to be right.
I have found that keeping the throwing hand protected is a pattern that needs to be built into muscle memory with LOTS of practice. One of the things I found effective was having my DD catch foam balls bare-handed, and making SURE that her throwing hand does what it's supposed to do until that action becomes the new "normal".
For kids moving to C, this is a common issue. We teach them to catch "two-handed" from the time they first start playing catch. Behind the plate, that habit continues unless they put in the effort to break it. When working with a new catcher, this is one of the first areas I focus on.
Behind the back pushes the bone in the shoulder forward and closer to the surface, and more susceptible to damage. Hand by the ankle leaves a little bit more flesh to take any abuse. Bruised muscles heal faster than bruised or broken bones.