From Thierry Bardini’s Bootstrapping:

“The ‘knee-control’ was an alternative design for the mouse that was supposed to be used under, rather than on the desktop. Movements of the user’s knee controlled the device and the bug on the screen, a sideways movement of the knee controlled the cursor in the horizontal dimension of the display, while an up-and-down movement controlled its vertical dimension…With a little work, we discovered that the knee offered even better control at slight movements in all directions. In tests, it outperformed the mouse by a small margin.” (pg. 112)

Man, that would’ve made laptops really weird.