The algorithm for regular NIBP cuffs is interesting. It's actually measuring MAP - mean arterial pressure - and doing a calculation based on that and heart rate

The algorithm for regular NIBP cuffs is interesting. It's actually measuring MAP - mean arterial pressure - and doing a calculation based on that and heart rate to approximate actual systolic and diastolic numbers. This is usually as or more accurate than a manual cuff where those pressures are directly detected (because there is no human error in hearing exactly when blood is flowing again in your artery) but fails badly for the small number of people who fall outside the characteristics of the groups that were used to develop the algorithm that converts MAP and heart rate into systolic/diastolic pressures. (The gold standard, which all other methods are compared to, is a direct measurement taken by sticking a needle in your artery and putting a pressure gauge on it. For obvious reasons this is avoided as much as possible.)