How do Abigail Williams' actions significantly affect the outcome in Act 3?

At this stage in the play, John Proctor's getting desperate. Not only is his own life in danger but so is that of his wife Elizabeth. He figures that the only way to save themselves is by getting Mary Warren to testify that the whole witch-craze is based on nothing but a pack of lies. Mary's quite a weak-minded, suggestible individual, and John uses everything he's got to try and intimidate her into telling the truth.
However, John hasn't reckoned on Abigail Williams. When it comes to intimidation, she's in a league of her own. She threatens Mary, making it abundantly clear what she will do to her if she dares to open her mouth in court. This is no idle threat; Mary knows just how nasty and vindictive Abby really is. Just one word from those lying lips of hers could be enough to send Mary to the gallows on a trumped-up charge of witchcraft.
Nevertheless, Mary bravely begins her testimony. But Abby's not done with her yet. She and the other girls start shivering as if they've been placed under a wicked spell. Indeed, that's precisely what they claim has happened; they say that Mary has bewitched them with a cold wind. The court proceedings denigrate into chaos, with Mary succumbing to the atmosphere of hysteria and accusing John Proctor of being the devil's man. Abby's attempt to intimidate Mary has worked, and in the process the last real chance for bringing this whole charade to a close has gone.