I think that the answer to this question is that she tries to fly. It is not clear to me if she is really trying to fly -- if she really thinks she can -- or if she is acting. But she gets out of bed and flattens herself against the wall. She says that she wants to go and be with her mother. When she is told that her mother is dead, she says that she will fly to her mother.
She goes to then window and opens it. The stage directions say that she raises her arms as if to fly and she gets one leg out the window before the others pull her back in.
So I think the answer you are looking for is that she tries to fly.
The literal answer is that Betty goes to the window and says she wants to fly to her mother. That is what is written for her character to do.
However, there is more to it than simply pretending to fly. Betty is scared. She knows that what she, Abigail, and all the other girls did in the forest was wrong and she is afraid of the consequences. By pretending to want to fly to her mother, she is perpetrating the hoax to make herself be seen as the victim.
Her want to fly to her mother also indicates that Betty feels like she is in over her head in this situation and needs the guidance and support that she would find in a mother.