The second move is for target acquisition and often precedes a pounce. The wide pupils let in more light, enabling a more precise pounce.

Why the cat's body is preparing to pounce is entirely situational. Maybe it's a hunt, maybe it's play, maybe it's just because the cat is being a little overenergetic derp. When you spend time living with a cat you usually learn to identify them pretty reliably. Especially the last one.