Starter button explaination

[rockfla]

Okay I need some help understanding starter buttons. I am suspecting my starter button "might' be my issue on my 39 Merc. I was under the dash as best my big head could fit under the small space between the floor board and the back of the dash but the bottom lip of the dash curing back still blocked a clear view of my starter button. Feeling behind the button I felt only one wire coming in (or out) of it? SO looking at Michael's page and the possible need to get a new "button"....the button he has listed shows ONE wire coming in (or going out) Pic's 1 & 2. MY simple mind is asking JUST how does a "single" wire work because to my simplistic thinking, the button is the "basic" connection FOR power in....power out...SO one wire in and one wire out (see pic #3).....how does the "ONE" wire button work???? OR how do I meter/continuity check my button to see IF it is actually "Buttoning"??? Even IF the button operates through "grounding" a single hot wire? the power has to come from the "source" ie the battery......HOW do it tell the starter to "START"???? Doesn't there have to have a wire to the starter/solenoid???