"Then try for me right now," Tidus says. "Hold your breath and count it, and tell me how long you go for, okay? No hurting your lungs over it. I wanna know how long you can do it without practice!"

Because you need to compare times! That's the fun part, helps you know you're getting better and improving. Clearing the lungs out first helps, but they can do that after--give Romeo an extra boost in time.