Capacity Down

Capacity Down (キャパシティダウン) is a device that produces a sound that can disrupt an esper. Developed by Therestina, it was purposely leaked to Skill-Out factions such as Big Spider, as a means to gather actual data on its effects against espers.

Principles
There are two versions of Capacity Down, getting more powerful against espers 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 espers, which is similar in principle to the Level Upper. The first incarnation of the Capacity Down was used by Big Spider, on their attacks against espers; 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 its 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. All versions of Capacity Down however, do not affect a level 0, and would only be heard like a high-pitched sound to them.

In essence, the AIM Jammer has similar effects to the Capacity Down but use different methods, where AIM Jammer uses electromagnetic waves to disrupt AIM dispersion fields, Capacity Down uses soundwaves to disrupt the brain operation of espers.

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 its 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 Arc
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
It is likely that Capacity Down is one of the anti-esper technology found in the Reformatory.

Trivia

 * Kamijou Touma has 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. Some fans theorize that his brain is different from a normal Level 0's but any esper ability he possess is negated by Imagine Breaker.