With each heartbeat, blood flows through the arteries and blood vessels. The amplitude of the waveform indicates the relative blood flow, which correlates with the level of vasodilatation/vasoconstriction at that point.
The BVP Head Sensor for NeXus is placed on the Arteria Temporalis. Make sure to prevent too much movement of the head to prevent artifacts.
Use the Temperature Sensor in combination with the BVP Head Sensor in order to monitor both temperature and relative blood flow.