This important feature prevents them from sliding backwards during the push-off stage of the stride.
The maximum push-off power of the technique demands that these skis have only a minimal amount of sidecut.
The lid is light weight enough for you to push off and pull back into place as you access the attic and it fits tightly enough to seal out energy loss.
The toe box has a wider design than most other running shoes, allowing for full flexion and maximum push-off during running.
This is especially true if you make the movement ballistic, i.e. push off from the bottom position like you were trying to leap into the air, which is often touted as an advanced form of the exercise.