How does Giles Corey get killed?

Giles Corey is pressed to death because he would not enter a plea in court. Elizabeth Proctor tells her husband, John, in act four that Giles knew he'd be convicted if he allowed the court to proceed (which it cannot if he refuses to respond to the charges against him). If the court proceeded, he knew he'd be convicted of witchcraft, and then his property would go up for public auction. By keeping quiet, he allowed his farm to be inherited by his sons. The court decided to "press" him in an attempt to get him to enter a plea. Elizabeth says,

Great stones they lay upon his chest until he plead aye or nay. They say he give them but two words. "More weight," he says. And died.

Although the details of Giles Corey's real life are somewhat different than the way Miller presents them in the play, his death is recorded here with some accuracy. He was, in fact, pressed to death for refusing to enter a plea in court, and pressing was absolutely a legal method for the court to use in such a situation. He would have been stripped of clothes and laid in a kind of shallow trough in the ground, a heavy wooden board placed atop him, and then large stones would be piled on top of it. The goal was to force him to enter a plea, though he never would. Giles even cursed the sheriff before he died, and many believe that the curse actually did work, as that sheriff (and a great many others since then) died of mysterious heart-related causes.

In act 3, Giles Corey interrupts the court proceedings by claiming that he has evidence that Thomas Putnam is influencing his daughter to falsely accuse citizens of witchcraft in order to buy the land of the accused after it has been foreclosed. When Deputy Governor Danforth asks Giles Corey to identify the man who overheard Thomas Putnam speaking to his daughter about George Jacobs, Giles refuses and is charged with contempt of court. Giles Corey is then arrested and thrown in prison after challenging the corrupt court.
In act 4, Elizabeth Proctor visits her husband in prison and tells him about Giles Corey's fate. When Elizabeth tells John that Giles is dead, he initially assumes that Giles was hanged. However, Elizabeth tells her husband that great stones were placed on Giles Corey's chest until he was crushed to death. Giles refused to answer or admit to witchcraft, so that his sons would inherit his land. Elizabeth tells John that Giles's last words were "More weight" (Miller, 135).