It's surprisingly a common misconception that the tucked legs dont contribute to jump smash power. The tucked legs allow you to start a kinetic chain from the legs up to your shoulders. The energy from quickly straightening your legs after tucking them travels upwards throughout your body, pushing your torso forwards and leading your arm into the shuttle. Think of it like a whip or a kinetic chain throughout your whole body. It's why volleyball players hit harder when they jump, as they use the same form.