..or better on what point it begins the feed ?

If i set the X-Feed on the overhead panel to X Feed i see nothing what change in the fuel panel, still the two main tanks are feeding.

So i tought maybe it begins if the main tanks have around the same value of fuel as the two outer tanks but the outer thanks still does not feed in this situation, so at what time the feeds gets started, or is my thinking or doeing completly wrong ?

cheers 😉

Switching to xfeed alone won't do anything since the system won't know which tank you want use.

If you want a specific tank to be drained you would have to switch the pumps of the other tank to "off"