Verbose is welcome.
when I teach this, I have students squat down first on that back leg and then spring forward. This gives a plyometric boost, and emphasizes where the power is coming from. Front foot slides, and then suddenly "brakes" w/o additional knee movement.
This is the bit I was getting at. If you're doing the move as a forward movement and not as a plunging movement (a la Toyama) then you've got to get that forward energy
from somewhere. If you get it by stretching out the forward leg then you're slower than if you get it by exploding off the back leg. This is also how we teaching sparring drills - the difference being that the back heel is lifted instead of stuck in the ground.
I think for the seisan lunging elbow it's important for that back leg to stay rooted because you're basically turning yourself into a battering ram - if that back heel is up and you make contact before you expect - you may bounce off. With that heel already in the ground you're able to maintain the structure for the power.
This technique was portrayed in an Anthony Hopkins film entitled "The Edge" awhile back. In the film they sharpen sticks and shove them into the ground and try to lure the bear to impale itself on them.
When training this move in Okinawa, the teachers were very focused on making sure you showed even pace and timing. That this move is actually rather graceful. I was told more than once that I "didn't need to show all my power" on this move, rather I needed to time it well. I think this has alot to do with not letting your upper body get infront of your center of gravity on the move. I think was I too focused on the lean and less focused on what my lower body was doing. I ended up with a half-stomp, half-slide. When Mr. Takara demonstrated the move it was definitely more of an accelerating glide that kept accelerating until he stopped.
Again - the difference between a stretch from the front leg vs a plyometric push from the back leg.