Fall Factor
Fall factor is the term used to describe the ration between the distance fallen and the length of rope that is available to absorb the fall.
The energy that is generated in a fall is directly related to the distance fallen. In this regard the best situation (if we can call any fall a best situation) is that there is a short fall at the top of the climb, with a long length to the belayer.
The running belay is the way to achieve this ideal situation.