Both excesses rupture many of the potato's cell walls, allowing many starch granules to escape from their cellular prison. These granules, which have become gummy during the cooking process, give your finished mashed potatoes a pasty, rather than a fluffy, consistency. Your potatoes will be airier if you mash them in a vertical rather than stirring motion - and if you don't perform that task until just before serving time.
Whatever you do, don't try to mash potatoes in a food processor. Its sharp and fast blades can rupture the cell walls so quickly and effectively that you'll end up with gluey potatoes in practically no time.
** Asian Recipes