By mere refraining from food, Upavasa is not done. Upavasa means close to the Lord. This is called upavasa . Vasa or Nivasa means place of stay or place to reside. So Nivasa with Krishna is upavasa. Stay close to the Lord. That is real upavasa.
One who fasts on Prabodhini ekadasi (when the Lord gets up), does not enter again into the womb of another mother. Therefore, a person should give up all varieties of occupation and fast on this particular ekadasi day.