Jettisoning metal
Occasionally, you may find yourself in a situation where your fleets are carrying more metal than you'd like them to be carrying. For instance, if you are a Trader, and you are taking advantage of your special ability to haul two units of metal for each ship, your overloaded ships cannot fire. Even if you are not a Trader (or your ships are not overloaded), you may not want to be carrying metal in a hostile situation, since loaded ships will receive twice as much damage as unloaded ships when fired upon.
If you are at a friendly world, you can simply unload the metal on the spot (unloading happens before combat). But if you happen to be at an opponent's world and you'd rather not give the metal to your opponent, then you may have no choice but to jettison the metal, which destroys the metal for good.