Actually, the 85 lbs thing is just a figure that they use. If I recall the actual newspaper article correctly, the structure itself doesn't weigh much more than 10 lbs, and the idea is that the more weight you put in the backpack, the more electricity it generates, as it provides more 'bounce'.
And the 'bounce' is controlled, to be sliding along rails, up and down. Some backpack manufacturers (according to the newspaper article) are already experimenting with the controlled movement thing, saying that once you get into a set gait, that the up and down movement actually helps make the backpack feel lighter on your back.
Quentin