Hi there, could you expand? You mean in my case approx. 85kg of my 90+ would go through the hand?? And via the arm into the shoulder? I do practice "soft ukemi" from some throws, but have always seen the hand as a means of orienting my body to distribute impact evenly, rather than taking impact itself.
This method is completely counter intuitive and makes little sense until practiced, which makes explanation quite challenging, but let me give it a try:
We practice breakfalls as a very tight, small roll, with the first thing hitting the ground/mat/floor being the palm of your slapping hand. For some reason I cannot put into words, this takes all the impact out of the fall. Your palm may sting a bit but it is as if your body is lowered to mat with little to no impact. It is quite amazing. Been doing it for 20+ years and have not been injured, even under some devastating technique (hope I did not just jinx myself). I also have several deshi in thier 50's whom use this ukemi.
But don't take my word for it, try it out!