Another tip that helps with preventing unwanted board pivot while flat based is to use a little torsional twist to keep both edges slightly engaged. It robs a slight amount of inertia as Paolo mentioned, but it is an effective way to prevent an unexpected edge catch. To do this, I will apply just a little bit of toe pressure on the front foot to engage the toeside edge. At the same time, I am applying about thsame force to the heelside edge with my back foot. So, 90% of the board is flat based, but near the ends, it twisted a little so that at each end, opposite contact points are engaged thus preventing any pivot in the board. You have to practice a little to get the feel of this, but its very effective.
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