Once upon a time, the Red Triplets were servants of various powerful people, bound to the life of the one that summoned them from "the outside". Their masters could also summon them to appear at their location at any time. However, when the triplets secured their freedom, they also learned to tap into that summoning power and use it on themselves.

Baku and Biri are the true experts of the power and usually work together to transport materials and even other people with them. However, Bayzil's got a lock on summoning style, as you can see.