Powerful glutes plus increased moment of inertia from an earlier extended knee = more force hitting the ground as long as limb speed is able to be maintained.
And since Weyand studies showed limb speed is not correlative to faster sprinting, maybe the earlier extension is allowing the glutes and hams to prep for ground contact earlier allowing them to be stiffer or preloaded prior to ground contact?
Just some thoughts off the top of my head!