This form is concealed within the Symphogear relics under 301,655,722 locks, and can only be unlocked under a high-level phonic gain. The X-Drive appears to only last for one use before the user's Symphogear reverts to their normal form at whatever level they had last naturally unlocked it at.
When the X-Drive is active, the Symphogear gains a predominantly-white color scheme and enhanced design, universally including the addition of angelic wings formed of colored light. In this state, all Symphogears, regardless of their normal abilities, are capable of unassisted flight both in the Earth's atmosphere and in space. They can also communicate telepathically with each other, and others such as Finé. All of the user's normal attacks can still be utilized, but at such a drastically greater level of both power and scope that destroying an entire army of Noise is an easy task, and even standing up to a fully-energized complete relic becomes possible. On top of this, the X-Drive can be used to bolster the strength of another wielded relic, such as Solomon's Cane or Durandal.
The X-Drive's particular form and strength is based off the current level of power the Symphogear has achieved before unlocking it, as Hibiki, Tsubasa and Chris all possessed radically different X-Drive forms when they unlocked them again in G compared to their forms in season 1.
In G, the X-Drive was created for Hibiki, Tsubasa, and Chris, alongside the new users Maria Cadenzavna Eve, Shirabe Tsukuyomi, and Kirika Akatsuki, using everyone on Earth singing Serena's song, Apple, together.