"Can you explain this? or do you have a link to a website detailing what he does? I've never heard of it."
I'm under-qualified to answer this but I'll give it a go....
As I understand it the use of the weights promotes relaxed, efficient body movement and extension. Because it makes things bloody hard work - your body learns to economize. Same principle as training the same technique for a few hours non-stop. Another way they can help is by increasing and thus he lighting the physical forces at work on your body when you perform an action, for example tenkan.
I might have completely missed the point. There's a good called Iron Balls and Elbow Power available through Ezra Shihan's organization site:
I happily put this plug in for two reasons: Ezra Shihan has personally been supportive of me over the years, and I regard him as one of the finest aikido instructors in the world (having seen many), who I would love to see better recognized by the wider aikido community.