During a siege, a food is throw over the wall by the defenders, obviously accompanied by some swearing. When the attackers see it, they think "these guys are not starving if they are throwing away food like that. Lets abandon our siege!"
I have heard stories like about a couple of places, and it is certainly the local legend of Monção, Portugal, where it is said that a woman baker called Deuladeu become the city hero in the middle ages by throwing the last bread, made from the last flour supply, over the wall, thus ending a Spanish siege. She is represented in the city coat of arms
Here is her legend in portuguese:
this is an approximate translation of her supposed speech:
To thee, who were unable to conquer us by arms, and who have tried to force our surrender by hunger, being ourselves more humane, and - why, thanks to god, being ourselves well supplied - and as we see that you are not well nourished, we send thee this succor, and we will send more, if you ask.
PS: There is also a green wine (Vinho Verde Alvarinho, the local delicacy) called Deuladeu, and she appears in the bottle.
and her name appears to mean "God has given her to us"