Adjustment of the headstay tension is controlled by the backstay and runners(if your boat has them). Pulling tension into the backstay draws the head of the mast back which adds tension to the headstay. On a masthead boat the amount of backstay tension is a 1 for 1 ratio. On a fractional rig that ratio decreases because some of the backstay force is used to bend the mast leaving less to control headstay sag. Adding a jumper strut to a fractional rig allows more of the backstay tension to translate into headstay tension.
The downside of adding backstay tension to the B&R rig is that it decreases the tension in the shrouds and diagonals on the mast.
If your rig is one that has no backstay then the headstay tension is controlled entirely by the cap shrouds and is for all practical purposes non-adjustable after your rig is tuned.
