Ati means excess: these points give excessive pain when they are hurt. The nine points form the grid through which energy spreads across the plot, so a column, heavy object, piercing or wall on them blocks it, just as a knot (gathan) inside an outwardly beautiful body can turn into cancer.
Like the inner nadis of swar vigyan, the land has inner channels; when they are blocked the residents suffer vata, pitta and kapha diseases. That is why ancient texts advise an aangan (courtyard) or open space in the centre, or an area kept free of regular stable activity and heavy items.
Every crossing of grid lines across the plot is a marma, and there are many; the nine ati marma matter far more. Cases were seen where a column or toilet on an ati marma left the owner with a long illness that lasted to the end, and each ati marma harms in its own way.