"Obviously, nothing takes away the ommander's authority to land immediately (overweight if necessary) if he/she feels the flight to be in immediate danger."
....bearing in mind that there can be factors other than weight to consider. On a 747-400, for example, CG is one of these (as there may still be fuel in the Horizontal Stabiliser) tank in the tail of the aircraft). You don't want to be too tail heavy for landing.
(Speedbird 744) Fuel dumping on the 747-400 is somewhat similar to that described by SLT for the Airbus. The dump control computer (A or B) is selected via a knob on one of the overhead panels, the desired remaining fuel qty is dialled in by the crew using another knob and the jettison valves are opened with two pushbutton switches (on the same panel), allowing fuel to be pumped from two nozzles in the trailing edge of the wings. The nozzles are just outboard of the Outboard Ailerons. The Jettison Valve switches are guarded with hinged plastic caps to prevent accidental activation.
Unwanted fuel is dumped from the Horizontal Stabilizer Tank, the "Centre Wing" Tank and the inboard (wing) Main Tanks by high capacity pumps in those tanks (not the regular "boost pumps" in the main tanks however, as these are required to maintain the fuel flow to the engines).
When the fuel level has reached the selected target, the dumping is automatically stopped ... at which time, the crew put the dumping controls on the overhead panel back to normal.
Note: Fuel Jettison panels may vary from 747-400 to 747-400. For example, some have a "MLW" selection which automatically dials in the correct amount of fuel to be dumped to ensure that the aircraft will be at or below Max Landing Weight.
Regards.
Q.