The transfer pumps supply fuel to the supply tanks via pipes, at a rate that is higher than the engines can suck it up, the excess fuel pores back into the main tank via 'holes' through which the transfer pipes run, above 160kg this means all tanks are essentially connected. Hence the fuel runs through the holes to the supply tank until this point.
At a certain flight attitude you can reduce the main tank further without transfer pumps.
Hard to explain without a diagram.
