We are only as good as our 'base of support' because the body works from the ground/bottom, up to counter the pull of gravity- especially noticeable in climbing. To be efficient in countering these effects of gravity, our posture (core) needs to be in a position that can produce downward force that is perpendicular to the 'ground'. If you are smearing on a vertical wall- your shin has to be at a particular angle between your foot (base) and your center of mass to produce downward force which will hold you 'up'. If that shin angle changes, you will slip in the direction that it gives to.