I was about to wire my guitar using this official scheme and I can't seem to understand it properly. I get that the signal is going through the black wire to the volume pot, but how does it get (and more importantly - return modified) to the tone one (lower on the pic)? I supposed there should be a circuit for the signal to go there and then return, but there's one wire connecting the pots and that's it, the other one is just ground.
Black is the hot wire from the pick up and it goes to two places; the volume pot and the tone pot. The volume pot ratios the hot wire and feeds it to the output jack. Note the green wire - it should be regarded as ground and it connects to both pot bodies and ground on the jack socket. Note also the cold end of the volume pot also connects to green via its body.
The tone control pot via a small capacitor shunts high frequencies from hot to ground. If the pot is turned one way it hardly has any effect.