Forget anything you know about inflatable traction kites. Powerful as they may be, compared to a solid wing, they are docile and slow, and they don't make power at the high angles of the flight window. That little solid wing could accelerate to 75kts in 12 kts of wind (9 deg AoA), and the momentary force generated was around 2.5kN peak (my calc, not measured). Because the force is generated so high in the flight window, it is not possible to dig your feet in, it just lifts you up and throws you downwind.
Back in 1999 I built and flew a solid wing kite as part of the development of my speedsail design. It was small (2.4m x 0.8m) and we flew it several times in 10-12kts of wind. It was brutal. Zero force at take off (I had to run upwind) then with the very low drag it would accelerate across the window so rapidly it was impossible to control (I had a three line setup, but it traversed the window in around 1 second on 40m lines, and I couldn't react fast enough). The force generated at these speeds was so great that I was snatched back down the sand for 20m or so, really viciously.
Not sure I'm understanding this right. In 10-12 knots of wind, that little kite dragged you 20 meters?
I was using a cruciform control bar, which in retrospect was a mistake, because as the wing shoots up it applies more and more angle of attack, which doesn't stall the wing as it was going faster and faster, until I was pulled off the ground and rotated by the control line, but the wing still continued climbing and arcing to windward until the AoA is lost and drag, plus me hitting the ground to leeward, prevent it going any further.
The photo is with the wing flying on very short lines (5m or so) in an attempt to limit speed and hence add some control, but it was like holding on to a bucking bronco and no better controlled, though less dangerous as the forces were lower. In the photo the wing is fluttering back to leeward having already overshot me, and looks like it is about to crash with so much roll. It was very hard to get pictures of it soaring, as the soaring portion happened so fast. Somewhere in the old tapes there may be some video.
I know very little, so it's not too hard to forget all I know about them.
Your comments make me wonder if there's some way to attach a hard kite to a boat instead of a human and still control it somehow.