I just read a book called The Angle of Repose. The angle of repose or the critical angle of repose or granular material is the steepest angle or descent or dip relative to the horizontal plane to which a material can be piled without slumping. It's the idea of how high can one pile the sand before it falls down. The author uses the concept to explain how far the character can be pushed before he crumbles. One has to hold on to the angle of repose. That is the moment before everything crumbles.