A running backstay is a rigging component on a sailboat which helps support the mast.
During beating or reaching, the running backstay in windward side is in tension.
Another function of the running backstay is to adjust the tension of the forestay to suit the sailing angle and sea condition.
In general, during a hard beat when the boat sails very close to the wind, the running backstay is tightened to increase the tension of the forestay.
Under these conditions, the draft of the genoa is brought forward reducing the drag.