What does John Proctor accuse Abigail Williams of in order to protect his wife from being hanged?
In Act 2, Scene 2, Mary Warren gives Elizabeth a poppet (a doll) as a kind of peace offering. Elizabeth thinks this is strange but accepts the gift. In Act 2, Scene 4, officials come to the Proctor home to arrest Elizabeth based on charges of witchcraft. They search the house and find the poppet. One of the court officials, Ezekiel Cheever, claims that the poppet has a needle in it and this proves that Elizabeth was using witchcraft to stab the doll and thereby torture Abigail. Then Mary admits that it is her poppet but the officials are not convinced. Proctor insists that Mary accompany him to the court to prove that the poppet was hers and that Elizabeth has no dealings with witchcraft.
They go to court and Proctor pleads his case, but the court is still unconvinced. In court, Abby claims that Mary is sending out her spirit to harm Abby. John knows this is a lie. And seeing how quick the court is to believe Abby and any other accuser, Proctor decides to admit to his affair in order to show that Abby has a motive for going after Elizabeth. John and Elizabeth had earlier concluded that Abby is jealous. Abby and John did have an affair and John ended it. Abby protested but John insisted. Therefore, John and Elizabeth believe (rightly so) that when Abby accused Elizabeth of witchcraft (using the poppet as "proof"), she was getting her revenge on the woman who stole John from her. Proctor divulges all of this to the court and says that Abby's behavior is "a whore's vengeance." So, John essentially accuses Abby of lying, using the poppet to incriminate Elizabeth, and he accuses her of being jealous enough to seek vengeance on his wife.