Related Words: Damper Tray Capstan,
Capstans are small screws with smooth, flat heads designed to push certain mechanisms into motion. Contrary to the common household flat-head screw used to secure materials together by sitting flush against a surface, capstans can be adjusted to sit at various heights in order to change the timing of its corresponding mechanism.
Capstans can be found at the end of the keys in both grand and upright pianos, although they may be used to regulate different functions. In grands, for instance, these capstans are used to change the height of the hammers (also known as "blow distance"), but in verticals, they are used to regulate the amount of lost motion between the keys and the wippens. You can also find capstans attached to grand piano cheek blocks, key beds, trapwork, and damper top posts for regulating key bedding and damper pedal timing.
Most of these capstans are made out of brass or chrome (both of which have their own advantages depending on their function), but they can be made out of other materials as well; Steinway uses wooden dowel-style capstans while Yamaha uses plastic for their upright key capstans. Regardless, you can identify a capstan by the small holes located around the circumference of the head so it can be turned from any angle using a long, thin capstan regulating tool:
One hexagonal-type capstan variation requires a capstan regulating tool with a claw tip: