After having led numerous military battles against the English during the Hundred Years' War, Joan of Arc is captured near Compiegne and eventually brought to Rouen, Normandy to stand trial for heresy by French clergymen loyal to the English.
On 30 May 1431 Joan is interrogated by the French clerical court. Her judges try to make her say something that will discredit her claim or shake her belief that she has been given a mission by God to drive the English from France, but she remains steadfast. One or two of them, believing that she is indeed a saint, support her. The authorities then resort to deception. A priest reads a false letter to the illiterate prisoner supposedly from King Charles VII of France, telling her to trust in the bearer. When that too fails, Joan is taken to view the torture chamber, but the sight, though it causes her to faint, does not intimidate her.
When she is threatened with burning at the stake, she finally breaks and allows a priest to guide her hand in signing a confession. However, the judge then condemns her to life imprisonment.
As the jailer shaves her head, she realises she has been unfaithful to God. She demands that the judges return and she recants her confession.
As more and more around her begin to recognise her true faith and calling she is permitted a final communion mass. She is then dressed in sack-cloth and taken to the place of execution. She helps the executioner tie her bonds. The crowds gather and the fire is lit. As the flames rise the women weep and a man cries out "you have burned a saint". The troops prepare for a riot. As the flames consume Joan the troops and crowd clash and people are killed. Joan is consumed by the flames but they protect her soul as it rises to heaven.

Question: What causes Joan to break down and sign a confession?
Answer: The threat of buring at the stake.
Alternative answer: she's threatened to be burned at the stake