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.

I don't know exactly, but using one, it has the sensation of using the trackpoint on a Thinkpad. The puck you push, pull, and twist to make the motion is a bit springiness. The harder to push it, the faster the motion happens in the software on screen. When you release, it springs back to neutral. The old ones (late 90's vintage) had trouble coming back to center, and there was a hot key to rezero it if it was starting to drift. The newer ones don't seem to have the issue.

It’s a knob that spring back to its origin when let go, that can move ~45 degrees or ~1/4” each axes(subjectively). It feels a bit like those old bobbing dashboard toy in hand.

Mechanically, it’s something like an Rx/Rz two-axis joystick with sliding knob part on top that can be slid in X/Y, pushed/pulled in Z and twisted in Ry as well. All axes are spring centered.

Imagine a 3-axis joystick that can also sense translation. So aside from pitch/roll/yaw you also get the linear up-down/left-rigth/forward-back axises.