There's a picture on the Wikipedia article: https://en.wikipedia.org/wiki/3Dconnexion and there's a patent you can look at: https://patents.google.com/patent/US20050172711A1/en?oq=2005...

It's flexible, and there are LEDs and linear optical sensors set up with an occluder in between that casts a shadow (or passes a light through a slit, according to the patent). The sensor detect where along the sensor the shadow or light falls, which indicates how much you have translated or rotated the control in a particular axis.