1when to buy it
The Traveling Merchant only lists it once the Eye of Cthulhu is dead, which drops it into an awkward window. By then you may already be pulling yoyos out of the Corruption or Crimson that are close enough in performance that paying gold here is really a question about whether you like the weapon class at all.
If you do, buy it. Yoyos hang where you throw them and keep hitting, which is excellent against anything slow or rooted and frustrating against fast fliers that leave the radius. Getting a decent one early is the cheapest way to find out whether the play style suits you before you invest in counterweights and string upgrades.
the name
It is named after a real yoyo. One Drop produced a model called CODE1, short for a community design experiment run with the YoYoNation forum, and the mark at the bottom of the tooltip is their logo. The sprite is also drawn with a more angular outline than any other yoyo in the game.