Great question! Let's see what a Sesei and expert in this system has to say
Sensei James Williams stated the following in Aikido Journal
"..Nami ryu is based on the Yoshida han Yanagi ryu, that is really the basis of it. It also has a strong influence from Kuroda Tetsuzan Sensei with whom I have been privileged to train with over the last five years or so. There are also influences that have come from Systema. Odd as it may seem at times, but because we're not technique driven-- we're principle driven--so Systema has much to offer. To find principles that are the same in an old classical Russian art as they are in ancient Samurai arts was a revelation to me a few years ago. And certainly has impacted me in the sense that Nami ryu is more fluid, more relaxed, more spontaneous, and more operating system driven and not as rigidly technique based as many other Japanese arts are. My personal feelings are that the Samurai arts were much more fluid and spontaneous when they were actually being used for combat. The current rigidity, in my opinion, has much to do with the change in attitude in regards to combat function and also the methods of transmission that are being used..."
I hope this helped. Also check out the links I included
Resources Interview with James Williams - Part One by by Stanley Pranin- Aikido Journal
N A M I R Y U A I K I H E I H O
On the 2004 The Bugei Sword Discussion Forum
Sensei James Williams talks more about his system;0)