The Wippen Assembly is the most complex part of a piano action and is responsible for the most precise and sensitive control of the motion of the Hammer to and from the string. Both the Balancier and the Jack are responsible for the actual pushing of the Hammer toward the string. The black areas visible above are the carbon coating on the Balancier and Jack in the areas where they push against the Knuckle (shown above).
The Hammer "escapes" from the Jack or from the Balancier depending on how the key is moved. Basically if the key is moved from a full stroke the jack does the pushing but if the key is moved from a lower position as in fast or very soft repetition, the balancier does the pushing. This dual pushing of the hammer is called "Double Escapement" and is the single most important innovation in piano action design.
The Repetition Spring keeps the balancier in position to be able to perform the above described repetition while the Repetition Felt Block is what the Jack rests against when the Balancier takes over to perform repetition. The Hammer Rest provides a soft landing place for the hammer after it has performed a particularly forceful blow and bounces back all the way down.
The Spoon is simply a stop against which the Fly Regulating Screw adjusts the at rest position of the Jack directly against the Knuckle (shown above). The Balancier Covering provides cushioning if the Balancier moves all the way up against the Hammer Flange (shown above) and the Support Cushion provides the curved surface against which the Capstan Screw (shown above) actually pushes up on the entire Wippen Assembly.
The Center Pin shown here is the one on which the entire Wippen Assembly pivots as it moves the Hammer.