What's this?

Headlight "dimmer switch."

When you turn on the headlight switch, it feeds power to the "common" terminal of that switch which is properly known as a SPDT or "Single Pole Double Throw" and some guys call these a "two way switch" but that is not the proper technical term

The switch has a center terminal to which headlight power is fed, and no matter which way it rests, power goes to either the hi or the lo beams. "Mashing" it of course switches to the opposite circuit

Section 8 of you factory manual, "electrical"