Ok I'll see if I can do a vid - actually this can be done with a wide grip or narrow grip. You may have to wait a couple of weeks when I get to that stage of my training cycle when I start to use those type of moves
Not much impact on the knee since when you land your leaning forward, lots of posterior chain stress though. Not the kind of impact I get from a vertical jump dropping down from 3 feet+ anyway

The landing is of a more slower cushioned type since you don't land and recoil, you basicly go straight down upon landing
Its done in a continously flowing move - I have used 135lbs on it, and I can get airbourne no probs. Its just a by product of the speed and accleration. Stronger guys can use more weight. And your lower back will well and truly feel it.
The thing is, when you jump, you have forcefully extend you legs and hips. Getting up on your toes is not the same. Its not the move so much, its the thought behind that counts the most - the CNS. ALthough with heavier loads, even if your attempting to jump you will probably get up to your toes anyway. But again its the thought of wanting to jump that counts, its just the load keeping you pinned to the ground