Re: bokken strike style
As Tony said, and as demonstrated in the videos he posted. Definitely not involving upper body tension in either case.
The exception is that I've been taught to do the L-shaped cut at any level, not just to the bellybutton; part of the point of the strike (so I thought) was that one could control the finishing point of the strike by extending asymptotically out to ground and simultaneously to any point in front of oneself, without clamping down on the sword and tweaking one's arms.
It sounds like the finish of the D-shaped cut is more of a drawing slash, as opposed to the more tsuki-like finish of the L-shaped cut?