Black Counterweight

Land a hit with a yoyo while one of these is equipped and a second small projectile launches from you, flies to whatever you just hit, then settles into an orbit around your character. It stays out there until you recall the yoyo, and it hits for the yoyo's full damage, so on paper you are running two yoyos off one weapon. Buy one from the Traveling Merchant or the Skeleton Merchant whenever either turns up with them.

The counterweight applies global invincibility frames when it connects, which is the detail that decides where it is worth using. Against a single target, its hits and your yoyo's hits compete for the same window, and you can end up trading a strong yoyo tick for a weaker counterweight tick. Against a crowd, the orbit sweeps things off you while the yoyo works somewhere else and nothing is wasted.

So it is at its best clearing events and invasions, and at its most questionable in a straight boss fight where every frame belongs to one enemy.

The orbit is tight by default and barely extends past your own reach, which limits how much of the screen it can defend. Fitting any yoyo string, Magic String included, widens it noticeably. Since you are likely running a string anyway to extend the yoyo itself, the two upgrades belong together and it is worth having both before judging whether the accessory earns its slot.

It also reverses its spin after every hit and every wall bounce. That is cosmetic, but it makes the orbit look erratic and it is easy to misread as the projectile losing tracking.

The six colors are mechanically identical and the black one is a twelve sided die, matching the real yo-yo accessories these are named after. Wearing two does nothing, and they do not stack with their own upgrades either, so one is the maximum useful number.

Whichever color you wear sets the projectile color, unlike the Strung Counterweight and the Yoyo Bag, which pick their own. You can override that by dropping a counterweight into a vanity accessory slot, and if you have several in vanity slots the lowest one wins.

Long term the plain counterweight is a stepping stone. It feeds directly into the Strung Counterweight, and the yoyo bag rolls the same effect into a single slot along with the other yoyo accessories, at which point there is no reason to keep wearing one.