This stage of the play marks a very disturbing shift from what has been "play" or "sport" with the girls dancing naked with Tituba in the woods to something that casts its dark shadow over all of Salem. Note how Tituba is pressurised into "confessing" and also how she is pressurised into denouncing others who are witches. Parris himself says to her: "You must confess yourself or I will take you out and whip you to your death, Tituba!", and then Putnam follows up directly by saying: "This woman must be hanged! She must be taken and hanged!" Clearly Tituba has a rather unfair choice to make, which will then go on to face so many others in Salem - she either confesses and denounces others or dies.
Note what Hale says to Tituba when she confesses and begins to say she saw others:
You are God's instrument put in our hands to discover the Devil's agents among us. You are selected, Tituba, you are chosen to help us cleanse our village. So speak utterly, Tituba, turn your back on him and face God - face God, Tituba, and God will protect you.
It is clear that turning to God also means there is the expectation that you will turn others in. As Abigail sees what is happening she herself "confesses" and turns others in too:
I want to open myself! I want the light of God, I want the sweet love of Jesus! I danced for the Devil; I saw him; I wrote in his book; I go back to Jesus; I kiss His hand. I saw Sarah Good with the Devil! I saw Goody Osburn with the Devil! I saw Bridged Bishop with the Devil!
Hearing this, the other girls begin to join in, denouncing other people in Salem, accusing them of being seen with the Devil, and thus the witch trials begin.
The girls are all confessing to whom might have seen the devil.