After baking soda is activated, generally by mixing dry and wet ingredients, the magic and a timer start. Once all of the baking soda has reacted with acid, it can no longer create those first bubbles from mixing. If you leave a mixed batter on the counter long enough, some of those initial bubbles will burst, deflating your batter. If you stir that batter at that point, you'll lose a lot more of the leavening effect.