Conversation so far:
How does season 7 of Buffy the Vampire Slayer start for Willow Rosenberg?
The seventh season starts with Willow Rosenberg in England, unnerved by her power, studying with a coven near Giles' home to harness it.
Why is Willow Rosenberg unnerved by her power?
Willow Rosenberg fears returning to Sunnydale and what she is capable of doing if she loses control again, a fear that dogs her the whole season.
Does Willow Rosenberg ever return to her home?
Yes. In the final episode of the series, Chosen, Buffy Summers calls upon Willow Rosenberg to perform the most powerful spell she has ever attempted.
Does Willow Rosenberg use her power during this season?
Yes. With Kennedy nearby, Willow Rosenberg infuses every Potential Slayer in the world with the same powers Buffy Summers and Faith have.

Question: What happens when she casts the spell?
Self-contained rewrite: What happens when Willow Rosenberg casts the spell?