4015 #40 Economy Chain Master Link for the 40-1R-10 Economy #35 Chain. 

Do not use this master link with any performance chain. 

The master link, or what is known as a quick-release link, is used to connect or quickly disconnect the roller chain easily. The master link makes changing lengths or replacing chains very convenient. However, if using this in a racing application, we highly recommend using a chain breaker tool to make permanent chain lengths. Otherwise, the master link may come apart.