Negación

is the term used for the fields used by Hollows to rescue fellow Hollows. Only the Gillian-class Menos have shown the ability to use it. Negación fields surround a target in a square beam of light that isolates the target from the dimension they are currently in, making it impossible to harm them. The field then pulls the target towards the menos that created the field.

Menos Gillian
All Gillian class menos can use Negación, but only if the target is outside of Hueco Mundo (Living World, Soul Society, etc.) They only use the technique to aid their comrades or fellow hollow, which during Aizen's escape were Kaname Tōsen, Gin Ichimaru and Sōsuke Aizen. General Yamamoto stated that once that beam of light surrounded them, it was over--there was no way to reach them as the Negación field is impenetrable.

Caja Negación
Espada have a special cube-shaped device called meant to simulate Negación fields, which are used to punish their Fracción if necessary. The cube, when used on a subject, traps them in an alternate dimension from which there is no escape; however, this assumes it's being used as intended. More powerful arrancar, namely the Espada themselves, can escape given enough time. The only occurrence of this negación field being used was when Grimmjow incapacitated Ulquiorra with it. Apparently the small cube device must be placed in the arrancar's hollow hole, as during their battle Grimmjow waited to use it until he could place it in Ulquiorra's chest.