TOPIC: Question: Necklace of the Zephyr

Necklace of the Zephyr allows you to cast a spell as a free action...do your hands have to be free to cast? As in, if I attack with a sling, normally, I would have to use a free action to swap for something else before I could cast a spell, but this necklace also needs a free action. Or does it work similar to medallion of mystic mouth; swap out-cast-swap back as a single free action?