Available once you have reached the boat that travels to Luca, simply use the blitzball, listen to Jecht's voice, and move the LS in the direction of the voice and press the matching button. I found that you had to be relatively quick to do this, no waiting or feedback to inform you that you've done a correct/incorrect position. You can retry as many times as you like if you fail.
You can return to the boat after doing the Luca storyline section, until you reach the Shoopuf.