There's nothing to really take a photo of. I got some 2mm bungy cord. Made a loop in one end and attached that to the handle of the knife. All you can see is a little bit of elastic looped through the handle and disappearing into the harness.
The bungy is 40-50cm long so it stretches out to a full arm length. I'm guessing if I was about to die and needed to cut something I would find the strength to stretch it out further.

If you poke around the knife holder you'll find a little gap on each side. I passed the bungy through that gap and tied the other end onto where the straps are attached (and the buckles and things). The excess bungy is pulled back inside that end bit under the fabric that covers the adjustment buckles.
That's it. The knife stays in place 99% of the time, including in quite vigorous crashes. It only comes out if I'm messing about in the water and bump the knife against the board.