The above video is really meant to be a clear demonstration of principles. It's actually a challenge for me as to how helpful it is sharing these clips openly on the internet, as outside of understanding the context of the practice, people can get completely the wrong idea about what's going on. That being said, the "tensing" that seems to be apparent is a result of uke maintaining intention and tori attempting to disperse and redirect while maintaining the point of contact.
For aikido people, some of the thinking behind this kind of training is well articulated in this blog, especially in the comments by George Ledyard re; static practice. I'm not suggesting that it's the same in terms of body organisation, but the approach and mentality is similar.
There is a concept in this training called "hari". It's a very important fundamental concept. It refers to a kind of "tautness" that runs through the body. That's not a good word, but I can't think of a better english equivalent. Hope this helps. It's very difficult to really get anything across via internet.