The Iwama weapons system has 7 ken and 20 jo suburi. In the linked video, the ken suburi are demonstrated. The first ken suburi is a shomenuchi. Could you post a link to a video which demonstrates the "first suburi"/"shomenuchi suburi" as you believe it should be done?
I don't spend enough of my youtube browsing time looking at Iwama folks to have such a thing for you.
Can you offer an explanation as to why the shoulders should lift up and rock forward at the end of a shomen cut? And why the tip of the bokken wobbles around like it is the end of a spring?
I had the chance to look at the video with sound last night and the thought occurred to me that the tension might not actually be in Saito Sensei's shoulder muscles, but rather in his whole body, as he is expelling a fairly forceful kiai as he completes those cuts. it still really looks like his sword and arms are separating from his center at the end though.
I just don't know what to make of this.