It is always used in conjunction with a means to determine at exactly what pressure the flow of blood is just starting, and also at exactly what pressure it's unimpeded.
Ihlala isetshenziselwa ngokubambisana ngezindlela zokunquma ukuthi ingcindezi yegazi iqala ukuphikisana, futhi yikuphi ukucindezeleka okungenakulinganiswa.