What is the purpose of the external jumper wire on a variable-speed reversing drill switch?

The external jumper wire on a variable-speed reversing the drill switch to form one-half of a cross-connection system.

Explanation:

A jump wire is also called as jumper or jumper wire. It is an electrical wire or a group of electrical wire in a cable which may or may not have a pin or a connector at each. It is normally used for the interconnection of the breadboard components or a test circuit, other prototype or with other components.  

The fitting of Individual jump wires can be done by the insertion of their "end connectors" inside the breadboard slots.  The purpose of using external jumper wire on a variable-speed reversing drill switch is to form one-half of a cross-connection system.