Re: opening the joints
I thought I would chime in with another beginner's perspective, in hopes that it will either a. be correct, and therefore helpful to others or b. be incorrect, and therefore be helpful to me (assuming someone chimes in to offer a correction):
I think both "extending" and "opening the joints" speak to connection. We want our tanden to be the "engine." In order to transmit the power from the tanden to whatever we are acting on, we have to arrange our body into a frame. Other than the tanden and legs, the muscles that are used are not providing power, they are holding the frame alignment in place.
If the frame is not shaped correctly, it will buckle under the load. Opening the joints means arranging the skeleton in a way that will allow it to cleanly transmit the force without buckling.
Tensing the major muscles will also induce buckling, although the mechanism is less obvious. Remember that force will always flow through the "stiffest" path. It's possible that tense muscles lead to less than ideal paths for the force, paths which pass through the muscles rather than the bones. Also, tension in the biceps, for example, will tend to move the frame into a less efficient shape. We want to send the force along our bones. Muscles that act perpendicular to the bones must be relatively relaxed for this to happen.
While forcefully extending the joints may be a good workout for the tendons, I believe it is possible to extend beyond the "ideal" for force transmission, leading to a loss in power. I don't know enough to speculate on the merits of this as a training tool, but it's likely that a very forceful extension might be a hindrance in actual application.
From a practical standpoint, a joint that is fully extended is also close to being hyperextended, which is a risk. You don't want to present an opponent with 99% of an armbar.