Nintendo's Wii Vitality Sensor works by using an infrared light and a photodetector to measure the concentration of hemoglobin in a fingertip.
That's according to a patent application uncovered and translated by Siliconera, which noted that under stress blood flow increases and the concentration of hemoglobin increases, meaning less infrared light reaches the photodetector.
"The Vitality Sensor measures a player's 'relax fluid'. This is calculated by measuring a cardiac cycle (R wave to R wave) over 100 pulses. Similar to a Brain Age, this number can be compared to the relax fluid of same aged persons," Siliconera explained.
Apparently the application also details a game where players have to adjust their breathing to help pilot a character through an environment where he's not supposed to touch the ceiling. The game would use the Vitality Sensor to identify inhalation and exhalation by measuring changes to heart-rate.
Nintendo president Satoru Iwata first unveiled Wii Vitality Sensor at E3 2009 but we've not heard much about it since. The device is still in development but its original 2010 target appears to have been long forgotten.
Earlier this year Nintendo registered a trademark for Wii Relax, but there's been no word on whether it relates to the Vitality Sensor yet.