(Hopefully it works for you; on my work network I have access to a number of journals through institutional subscription and since I'm at work right now, I can't tell right now if that link works for everyone.) I particularly like fig 2. I also found it interesting that efficiency for the SDK was stated to be significantly better than freestyle.
I think there are two main reasons the downbeat is more effective: you engage the powerful quadriceps, and the flex in the ankles means more of the force goes to forward movement. That's why ankle flexibility is SO crucial: I think just a little increase in flexibility can dramatically increase your forward propulsion. If you look at underwater video of Phelps doing SDK, his feet are like a pair of flippers. (Sigh...maybe even more than height, I've always wished to have bigger feet than my puny little 9 1/2s.)
I injured my knee recently (slipped on ice) and for a couple days it hurt to hyperextend. What that emphasized to me was how much I depend on that in my SDK. When I was just cruising it was not a big deal, but when I really tried to kick fast, my knee hurt a lot during the less powerful "upbeat" portion (which is actually a downbeat on backstroke, but whatever).
It made me think about the role of the upbeat in my SDK. Maybe it is important not so much for its propulsion but for kick frequency: I was emphasizing it just to get my legs/feet back for the powerful downbeat part of the kick.
Fritz, I think you are absolutely right about the advantage of being able to see the water on backstroke SDK. It really allows you to time your breakouts better (I switch to flutter kick just before the breakout, for instance). I also think I make subconscious alterations in hands/body to keep in deeper (faster) water based on visual feedback.
One other issue that I think is really important is head position. Ande mentioned this elsewhere, about a "head break." A lot of coaches teach to have your hands in a tight streamline BEHIND the head, but I think that is incorrect. I was playing around in a 50m pool with fins, trying different head positions: the advantage of doing this with fins is that you are going so fast you can really feel the difference when your head is acting as a brake. The most efficient position for me was having your head between the arms, which were maybe JUST behind the ears.
I also think it is important to hold the arms very rigid in the streamline, not just for hydrodynamics but because it gives your kick more leverage.
What all of this brings home to me is the importance of practicing SDK for technique, speed and endurance. And having a race plan -- how many kicks off each wall -- that you train toward. It really is a 5th stroke. (Or, in my case with my lousy breastsroke, a 4th stroke.)