More than likely because he is in a custom ground plug. I can do the same thing in my 150 Lange plugs that PJ Dewey made for me. They are ground for my feet and my feet only, so the fit is conforming even with the buckles loose. I can experiment with different buckle tension and my feet stay put. However, I can't get the same effect with any of my off-the-shelf consumer boots that are shaped for to fit as many feet as possible. I need a certain buckle tension in those, or my feet will start to slip around.
PhilpugNotorious P.U.G.