While the siege was ongoing, she dressed in armor and rode about the town on a big warhorse, encouraging the defenders. But when she saw an opportunity, she took it:
And now you shall hear of the boldest and the most remarkable feat ever performed by a woman. Know this: the valiant countess, who kept climbing the towers to see how the defence was progressing, saw that all the besiegers had left their quarters and gone forward to watch the assault. She conceived a fine plan. She remounted her charger, fully armed as she was, and called upon some three hundred men-at-arms who were guarding a gate that wasn’t under attack to mount with her; then she rode out with this company and charged boldly into the enemy camp, which was devoid of anyone but a few boys and servants. They killed them all and set fire to everything: soon the whole encampment was ablaze.It's hard for us today to celebrate the killing of boys and servants by professional soldiers, but it occurs regularly in the Hundred Years War if they are present as a part of the army's logistical train -- doubtless you remember such a scene from Shakespeare's Henry V. They were performing an important part of the war effort, and as such they end up taking a soldier's chances.
When the French lords saw their camp on fire and heard the shouting and commotion, the assault was abandoned as they rushed back in alarm, crying: “Treachery! Treachery!” The valiant countess, seeing them alerted and the besiegers streaming back from all sides, rallied her men and, realising there was no way back to the town without grave loss, rode off in another direction, straight to the castle of Brayt, some four leagues away.
In any case, Jeanne and her party escaped the French. She returned after five days with reinforcements, and was able to re-enter her city. She continued the defense until the arrival of English aid, which routed the French. Eventually her son would become the ruler of Brittany.
Jeanne went on to fight in a sea battle on her way to England to obtain further aid for her people. Escaping from the French fleet to the harbor of a nearby town, her forces stormed and captured that city and used it as a base for further war against the French and their preferred candidate. She was an important leader of her husband's faction during his imprisonments, of which there were two, and after his death. Her son eventually won the throne, although it is not clear if Jeanne knew it. Later in her life, for reasons we do not know, we are told that she went mad. The English put her up in a castle under the guardianship of one Sir William Frank, a knight of Edward III's.