Mary is at first terrified...all the girls are in terror of Abigail since she has such a manipulative hold on them. Mary begins crying and pleading with Abigail not to do what she's doing. Abigail "sees" a yellow bird in the rafters into which Mary has "sent her spirit". The other girls follow along, as Abigail knew they would. In their defense, they didn't actually see anything, but they believed they saw something, so the entire episode for these girls was very likely true to them. Fear is a powerful motivator, and Abigail was an intelligent girl who played to the community's fears.
Abigail begins this charade when Mary can not force herself to faint as the girls have done time and time again in court, saying, "I--have no sense of it now, I--"
Abigail also feels she must discredit Mary Warren and John Proctor since in this same scene, Proctor has called Abigail a "whore" admitting that he has "known her" and this is the reason the girls have gone crazy blaming all the good women in the town for witchcraft in order to get their husbands.
Danforth orders weak Mary Warren to "draw back your spirit out of them!" Mary becomes overwhelmed by the girls' "utter conviction" and believability, begins to whimper, and turns against Proctor. She runs to Abigail, who takes Mary in her arms as to say, "All is forgiven, Mary". Mary accuses Proctor of making her sign the Devil's book.
Despite Proctor's insistence that Mary reveal the truth about Abigail's manipulation of the other girls and her own pretending to see spirits. When Abigail begins her performance of shivering and pretending to see Mary as a bird in the rafters threatening to attack the girls and claw them, the other girls quickly follow her lead. Mary at first pleads with Abigail. The peer pressure, however, is too much. Mary is only one girl against many, and she was described by Miller as "subservient, naive, lonely" early in the play. She cannot successfully fight the stronger Abigail, and as a result, to save herself, Mary turns against Proctor, declaring him "the Devil's man."