On Google I get some Wikipedia extracted information that says:
"The Valve Index Controllers have a joystick, touchpad, two face buttons, a menu button, a trigger, and an array of 87 sensors that allow the controllers to track hand position, finger position, motion, and pressure to create an accurate representation of the user's hand in virtual reality." with a link to Wikipedia: https://en.wikipedia.org/wiki/Valve_Index
See the first page of results for DDG's search on how valve index works:
https://duckduckgo.com/?t=ffab&q=how+valve+index+works&atb=v...
Compare to Google: https://www.google.com/search?q=how+valve+index+works