Ok, i think i get it now RG! I've gotten into the habit of thinking that there is a connection to physical ground / dirt when looking at schematics / thinking theoretically, through the guitar cable or something, but of course there isnt, the circuit still works, with or without a guitar cable plugged in. Or that, if the battery negative is removed from the circuit, it can just use any negative (like the negative from the DC jack for instance) like a rubbish bin or something. But as you have explained, it doesnt work like that. You have to have a circuit from the 2 terminals before power will be supplied.
Thanks for the explanation!