The reserve setting on your petcock allows you to draw fuel from the bottom of your gas tank. During normal operation of your motorcycle, you should set your petcock to the “on” position. This will allow fuel to flow from the tank through the top portion of the petcock and out to your fuel system. When you move the petcock selector to the “reserve” position, it allows the fuel to flow from the tank through the bottom portion of the petcock and out to your fuel system. This reserve function allows you to get to the next fuel stop before you run out of gas.