In Act III, scene 3, Mary is mocked and mimicked by Abigail and the other girls. Danforth wants the truth, which Mary tried to give, but she sees that she is slowly sinking on her own without Abigail on her side. She quickly "jumps ship" and claims that Proctor is the evil one who is trying to overthrow the court. So instead of helping Proctor and giving the court the truth of what happened, she quickly changes sides and does whatever it takes to save her self. This selfish act turns the tables on both John and Elizabeth.
Prior to that moment in the courtroom, Danforth wanted Mary to "fake her fainting" if that was what she did before. That's how she explains the way the girls acted before. She was unable to do it. (I'm not sure which section you are questioning).
In Act 3 of “The Crucible”, John Proctor brings Mary Warren to the court and judges to testify that the girls were in fact lying that that when she and the girls “cried out” that people from the village were witches they were lying. When questioned by the judges about “crying out” against these people, Mary Warren tells them that she was basically caught up in the excitement of everything and since the other girls were doing it, she felt that she had to do it too. Then, when she is asked to show the court how she pretended to faint, she can not do it, explaining that she has “no sense of it now”. She continues by telling them that she can not do it for the same reason that she was no longer crying out against people. Following this episode in court, the other girls begin to accuse Mary of witchcraft and when this happens, Mary gives up on trying to tell the truth and then accuses John Proctor of being a witch.