A clicky-bottom pen is a ballpoint pen with a button on the bottom that makes the writing tip extend from the hard plastic body of the pen. The pen consists of a body and a shaft.
The shaft of the pen has a metal spring and a plastic ink tube. At the end of the ink tube is the ballpoint. The ballpoint is cone-shaped and made of metal with a small opening on the end. Inside the ballpoint is a ball which plugs the hole. The purpose of the ballpoint is to allow a small amount of ink out of the tube as the ball is turned by moving it across paper. The spring is used to hold the shaft in the correct position against the body. This helps provide a satisfying click when the button of the pen is pushed.
The part of the body that acts directly to move the shaft is between the clicky-button and the shaft. The button on this pen is screwed onto this piece which then fits around the end of the shaft. This piece is shaped with inclined planes around the edge like a screw so that when the button is pushed it moves down the body, turns, and is locked in a new position against the body. As it is attached to the shaft and thereby the point, the shaft is locked in a new position farther down the body and the ballpoint is extended outside of the body.
The rest of the body is designed for aesthetic and orthopedic pleasure. On the end of the body with the button there is a pocket clip. This is made of a flexible plastic and shaped like a shallow hook with the intent that you can clip it to your shirt pocket. This pen has a rubber grip near the tip so the pen is more comfortable to hold while writing.