Abstract: The present invention describes a mechanism and method for retracting an instrument. In a rearward position, a sliding weight aligns an elongated assembly with a rear canal allowing the assembly to slide backward into the canal thereby retracting. When the weight slides forward, it collides with a protrusion on the elongated assembly forcing the elongated assembly forward and pulling the rear of the elongated assembly out of the rear canal. Once the elongated assembly is pulled out of the rear canal, it is unlikely to spontaneously return to the canal and contacts an internal shoulder which locks the elongated assembly in a forward position. Alternatively, means are provided to lock the elongated assembly out of the rear canal when in forward position. Preferably the weight is moved between the forward and rearward position by shaking.