This means that one should always shoot broadheads (preferably fixed blade) to ensure that the arrow will fly correctly with them. If it is flying well with broadheads and impacting the same as the FP' s then the spine is most likely OK. If, however they impact differently than the FP' s after the bow has been tuned, then the spine is the first thing I would investigate. If the broadheads act finicky and are very sensitive to form, the shafts may be on the borderline. Turning the poundage down or going to a slightly stiffer shaft should correct the problem.