In its current iteration, the Ulfberht+ has two methods of pointer input: a keyboard mod key, and a handheld bluetooth trackball. Bluetooth is useful--even viatal--due to the limited number of USB ports available for cords or dongles. The fact that it's handheld makes it easier to use in situations where work surface is at a premium, such as in a car or on a plane.

This device is quite ergonomic and feature-rich, but falls down in one vital area: while it has a "center click"--that is, a scroll wheel which can be depressed for panning input, the position of the scroll wheel relative to the primary trackball means that the cursor can't actually be moved while the scroll wheel is depressed, since both must be actuated with the thumb.

This is important, since for much of the work I do, whether it's CAD, GIS or graphic design, the center-click pan feature is indispensable. 

The mouse in question is similar in form factor to my Oculus VR controllers, which have an intriguing feature--a button on the side which can be activated by the middle finger, usually simulating a grabbing action in games.

This control method seemed intuitive and plausible to implement in the trackball...so I did. I opened up the trackball, soldered a new micro switch in parallel with the existing one that detects the scroll wheel being depressed, drilled a small hole in the side of the casing and used a 3D-printing pen to plastic-weld it in place.

The result is clean, practical and sturdy--you can see the small plastic nubbin just under my middle finger on the side of the device, below the power switch. It works flawlessly and looks practically OEM. Now I can CAD on the go!