How The Upright Piano Action Works

The upright piano action works to similar the grand piano action.

When you depress the front end of a key slowly, it pivots on the balance rail and the back end goes up.
The key lifts the sticker and wippen. The wippen pushes the jack, which pushes the hammer butt.

The hammer butt pivots on its own flange and moves the hammer toward the strings.
When the hammer is halfway toward the strings, the spoon engages with the damper lever, lifting the damper off the strings. When the hammer is almost to the strings, the jack toe bumps into the regulating button.

As the wippen continues moving up, the jack pivots on its flange and slips out from under the hammer butt.
The hammer continues under its own inertia to the string, hits the string, and istantly rebounds. The backcheck catches the catches and holds it in position while you hold down the key.
When you release the key, the wippen drops, and the backcheck releases the catcher. The butt spring pushes on the hammer butt until the hammer shank returns to its resting place on the hammer rail cloth.
The damper spring returns the damper to the strings, and the jack spring returns the jack under the butt, ready for the next repetition.