Question

what does katniss do as prim is about to climb onto the stage to accept her fate? why is her action shocking?

Answer 1

Katniss darts in front of her and volunteers to take her sister's place.  It's shocking because people don't normally volunteer.

hope this helps :)

Answer 2

Katniss volunteers herself instead of her sister. This is shocking because Katniss knows this is her fate, but she protects her sister by accepting her own fate. She doesn't want her sister to die.