Ultimately headstay sag is controlled by tension on the headstay_.more tension helps to minimize and negate sag. The backstay can help to add to the tension on that backstay, however there are also rig tuning ideas that can help to further maximize the abilities of the backstay adjustment.
Specifically on a swept fractional rig if you set the uppers and lowers to equal tension then you will minimize the mast bend and create a stiffer mast overall_.a stiffer mast will help to ensure that more load is transferred to the headstay as opposed to mast bend_that is not to suggest that the mast will not still bend, it will, only to a lesser degree.
If any of you are in doubt next time you have your foil or furling extrusion removed attach a professional model Loos Gauge directly to your headstay and watch the different headstay tensions you can achieve with the backstay applied at 50%, 70% and then maximum. You can also leave the backstay adjuster applied at miximum and relax the lowers and you will see that indeed headstay tensions does in fact change much as your mast bend characteristics will also change.
