Capacity Down

Capacity Down (キャパシティダウン) is a device that produces a sound that can disrupt an ability user. Developed by Therestina, it was purposely leaked to Skill-Out factions such as Big Spider, as a means to gather actual data on it's effects against espers. There are many versions of Capacity Down, getting more powerful against ability users each time another one is introduced, the equipment used to manage the device and produce the sound also gets larger. According to Hebitani Tsuguo, Capacity Down interrupts the brain operation of the ability users, which is similar in principle to the Level Upper.

Capacity Down versions and it's effects
The first incarnation of the Capacity Down was used by Big Spider, on their attacks against ability users; the equipment used to produce the sound was large enough to fit a back of a van (specifically, the van used by Big Spider). During use, Capacity Down would render Shirai Kuroko incapable "jumping" or teleporting, and Misaka Mikoto unable to properly handle her electric powers; they would also feel a slight pain in their heads or may even lose consciousneess during it's activation.

Capacity Down would be used again by Therestina, this incarnation is much larger compared to the previous one, and can easily bring a high-level esper such as Mikoto, to her knees. Control of one's ability is limited as well as one's body movement.

Another version of Capacity Down is used in Toaru Majutsu no Index, here - Capacity Down can even affect Accelerator, and is highly dangerous, it can make the ability user self-destruct by making the ability go out of control when one tries to use one's abilities during it's activation. The higher your ability Level is the more likely it is fatal when you use your powers, though Tsuchimikado Motoharu says only those above Level 3 are in life-threatening danger.

All versions of Capacity Down, however, do not affect a level 0, and would only sound like a high-pitched sound to them.

Big Spider Arc
Capacity Down first makes its appearance in episode 15 of the Toaru Kagaku no Railgun anime. It was first seen being used by Big Spider against Kongou Mitsuko, quickly fainting after it's use, and later against both Shirai Kuroko and Misaka Mikoto. It was destroyed in the next episode by Mikoto's Railgun, anticipating that Big Spider would it against them again.

Poltergeist Incidents
During Mikoto's first confrontation with Therestina, the latter uses MAR Base Research Laboratory's sound tower against her, crippling her movement severely, and easily besting her. Therestina also reveals to her that she built it, and used Skill-Outs as test subjects for it.

Capacity Down is last used during the final episode of the anime. It severely cripples Mikoto's group before Saten Ruiko, who is a level 0, destroys the control system for it, allowing Mikoto to finish-off Therestina.

Toaru Majutsu no Index
Capacity Down makes it proper novel debut in volume 15 of the Toaru Majutsu no Index light novels, when GROUP enters District 10's reformatory to save Awaki's old allies from BLOCK. It is said among the 25 anti-esper technology they have guarding the reformatory Capacity Down is one of them, though it is not referenced by that name.

In Volume 19 a upgraded version of Capacity Down is introduced, when Director Shiokishi, the one trying to destroy GROUP in that light novel, installed it in his military base. Unlike the ones in Volume 15 where espers only risk going out of control, here if Accelerator uses his powers it is almost guaranteed to make his lose control and self-destruct. To prevent that happening Accelerator hurled cars at the building to collapse it down on itself, destroying the Capacity Down machines before storming in to fight.

Trivia

 * Kamijou Touma have never been under the effects of Capacity Down as of Volume 22 of Light Novels, so it is unknown if his brain can be affected, with some fan theories on his brain is different from a normal Level 0's but any esper ability he possess is negated by Imagine Breaker.
 * While Capacity Down machines have been used in the Index plotline, it was never referred to with the name 'Capacity Down'.