The main ability that doubles the power of secondary gear abilities is Ability Doubler. In Splatoon 2, it is the signature primary ability of the Splatfest Tee, making each of the shirt’s three secondary slots count for twice its normal effect.
Ability Doubler is easy to confuse with abilities such as Special Saver, Quick Respawn, or Ink Saver, but those affect special-gauge retention, respawn timing, and ink consumption respectively. Ability Doubler does not improve a weapon directly; it changes how the clothing’s secondary abilities contribute.
One important correction to the wording is that Ability Doubler is not generally an ability exclusive to all Octo Expansion gear. It is specifically associated with the Splatfest Tee and its later replica, while Octo Expansion introduced many other clothing items and abilities. The ability’s distinctive role is tied to Splatfest clothing rather than to Octoling-themed gear as a whole.
The ability became strategically valuable because it lets players build unusual combinations of secondary effects on a single shirt. That makes the Splatfest Tee useful for experimenting with compact gear builds, even though the shirt is normally available only around Splatfest events.