Forum:How does Samus fire the Grapple Beam in Metroid Prime?

I was writing a sort of novelization of Metroid Prime (for fun). I got to the point where Samus is using the Grapple Beam to escape the Orpheon and I couldn't figure out how to explain how Samus uses the Grapple Beam. I thought about it and realized that I didn't know how she triggers it to fire. Could anyone provide me with an explanation, whether it be a theory/headcanon or actual headcanon? I would greatly appreciate it.