A pointing stick (or trackpoint, also referred to generically as a nub or nipple) is a small analog stick used as a pointing device typically mounted centrally in a computer keyboard. Like other pointing devices such as mice, touchpads, or trackballs, operating system software translates manipulation of the device into movements of the pointer on the computer screen.
Unlike other pointing devices, it reacts to sustained force or strain rather than to gross movement, so it is called an "isometric"¹ pointing device. IBM introduced it commercially in 1992 on its laptops under the name "TrackPoint" and patented an improved version of it in 1997 (but the patent expired in 2017).²

The pointing stick senses applied force by using two pairs of resistive strain gauges. A pointing stick can be used by pushing with the fingers in the general direction the user wants the pointer to move. The velocity of the pointer depends on the applied force so increasing pressure causes faster movement. The relation between pressure and pointer speed can be adjusted, just as mouse speed is adjusted. On a QWERTY keyboard, the stick is typically embedded between the G, H, and B keys, and the mouse buttons are placed just below the space bar. The mouse buttons can be operated right-handed or left-handed due to their placement below the keyboard along the centerline.

The cap is red on ThinkPads, but is also found in other colors on other machines. It may be gray, pink, black, blue, and green on most Toshiba laptops produced before the 2000s. Button configurations vary depending on the vendor and laptop model. Pointing sticks typically have a replaceable rubber cap, called a nub, which can be a slightly rough "eraser head" material or another shape.

Fetched from https://en.wikipedia.org/wiki/Pointing_stick

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¹ WO2001014957A1 - Control stick for a finger pointing device - Google Patents. 2001-03-01. https://patents.google.com/patent/WO2001014957A1/ko

² US6115030A - Trackpoint device - Google Patents. 1997-12-18. https://patents.google.com/patent/US6115030A/en