Biomechanics are simple -- The further away the weight that needs to be moved, the harder it is to move that weight. On a hockey skate, the runner is the furthest away from the core muscles of the legs that power your stride. Therefore, weight savings in the runner are amplified. At full extension of the skating stride, taking 1 gram out of the runner is equivalent to removing 4 grams from the boot. Those precious grams are needed in the boot to provide support for explosive energy transfer.