I have seen two configurations for running backs. One with a tang just below the uppers, applying headstay tension. The more common for controlling mast "pump" in a steep swell has the upper tang just below the spreaders and is not used to alter shape so much as to prevent the infamous fatigue at the spreader fastener line. All I have used is the latter which is applied going to weather and then just enough to prevent mast movement.
Doug.
