Hi i am big fã of h2o i want to ask what happen to Rikki in season 3 at the episode "Dark side" i don't understand

I am a big fã too. I think what happens is since everybody is hiding from the tentacle, when she try's to destroy it listens to her. She has control over it. But the other sereias don't.

I want to know to I didn't really understand it that much either please help