This is context for the people that don't know World Trigger. This is verbatim from the wiki and fanwiki so I do not own ANYTHING HERE!
In Mikado City (三門市, Mikado-shi) (280,000 inhabitants), a "gate" to a different world suddenly opens one day. Monsters called "Neighbors" (近界民ネイバー, Neibā, lit. "Near-world people") start appearing from the gate. Humans are overwhelmed at first when their weapons are found to be useless against Neighbors, until a mysterious organization appears that is able to repel the Neighbors' attacks. The organization is called the National Defense Agency, or "Border", and has appropriated the Neighbor technology called "Triggers", which allows the user to channel an internal energy called Trion and use it as a weapon or for other purposes. By activating a trigger, the body of users is replaced by a battle-body made of Trion which is stronger and more resistant.
A Trigger (トリガー, Torigā) is a product of Neighbor technology that manipulates and is powered by Trion. Despite being often used for combat purposes, Trigger technology also has other functions, and in fact it is regarded as the cornerstone of the entire Neighbor civilization. Combat-oriented Triggers have massive firepower, and are the only weapons capable of damaging and even destroying Trion Bodies.
Triggers can differ greatly among nations. Unlike the more powerful Black Triggers, which are created through the self-sacrifice of a person with high trion, normal Triggers are produced through technology. Earth developed its own Triggers after its contacts with the countries of the Neighborhood. The organization responsible for manufacturing Triggers and fighting off trespassing Neighbors is called Border.
To activate a Trigger, it is necessary to touch the Trigger Holder and give a clear signal of one's intentions to switch into a trion body. Border agents and trainees often use the vocal command "Trigger on" (トリガー 起動オン, Torigā On, Lit. Trigger start-up ?), whereas Aftokrator users declare the name of the Trigger. Black Triggers have been activated both verbally and non-verbally. Border's Triggers can be switched off at any given time, usually with the command "Trigger off," but none is strictly necessary. Aftokrator soldiers do not use any vocal commands to do so.
Normal Triggers can be activated by anyone, provided they have enough trion as even Operators can use trion bodies. Most, if not all, Triggers substitute the user's body for one made out of trion, storing the original body within the Trigger Holder. Trigger Holders are sent to the same dimension as the flesh and blood body upon activation, which is located somewhere in the Neighborhood. Trion bodies grant a Trigger user an increase in strength, speed and durability. Any wound inflicted to the trion body will cause a more or less severe leak of trion, but no physical damage to the user's flesh and blood body. When too much trion is lost, or when certain parts are destroyed, the trion body disappears.