Crossing the Threshold is the 6th step in the Hero's Journey. For the heroine of your story, the crossing may be involuntary. An external force may threaten her (think tornado, hurricane, earthquake). Someone seeking to harm her may kill or kidnap someone she loves. Or she may unexpectedly find herself in a situation with limited options. Dorothy in The Wizard of Oz crosses the threshold when the tornado whisks her out of Kansas and into Oz.
Internal pressure can instigate change as well. Standing up to a bully, feeling the need to achieve, following your own course despite what others say are all examples of choices that set your heroine on a different path. In Finding Nemo, Marlin (Nemo’s dad) faces his fear of the big sea and goes after the boat to find his son.
Whatever the circumstance, this is where your heroine commits to the adventure. Any fears or reticence are now put aside. She’s dispensed with her ordinary world, heard the call, perhaps refused it, and found a mentor to guide her on her journey. But now is the time to take that step into the unknown.
Of course, that step into the unknown may not be smooth. Your heroine's Mentors (the friendly helpers) could be offset by Threshold Guardians that create obstacles. Consider the father who believes his little girl is better off being a wife than a corporate president. The boss who keeps passing her over for a male counterpart who’s nowhere near as talented. Or the “secret helper” who may test the heroine to determine her worth and readiness. Getting past those Guardians might take physical strength, mental intelligence, or emotional bravery. Remember Mr. Slugworth from Willy Wonka? At the beginning of the movie you think he’s the villain, but he turns out to be an ally in the end.
Crossing the threshold can be rough. It requires special courage, an act of faith. Not only does the heroine have to commit to the crossing, she must trust she’s doing the right thing. And if a rough landing is involved, she has to keep on going. Moving from the ordinary world to the unfamiliar will take some adjustment.