- Pull-back cars store energy in a spiral spring by winding it through a gear system, invented by Darda in 1970.
- The gear train allows kids to use modest force to generate high torque for winding, and gears also enable speed multiplication for fast movement.
- In drive mode, the spring releases energy to propel the car forward, while in wind mode, pressing the car connects gears to wind the spring.
- The spring is wound by rotating two spring gears in opposite directions, and gears always rotate in opposite directions when meshed.
- The mechanism prevents infinite winding due to notches in the spring gear, ensuring the spring does not break from over-winding.