NSET Brane

The NSET brane is a 6-dimensional brane that exists alongside the 6-brane comprising Known Hyperspace. It appears inhospitable to transhuman life, and it remains unknown if any life can exist there, if it could be traveled to. NSET stands for negative stress-energy tensor, for which the NSET brane can be "called upon" to manifest sufficiently large gravitational sources in our hyperspace in order to travel FTL or propel spacecraft without rocket engines.
For this reason Metric Drives, and metric technology more broadly, require the NSET brane to function. Stress-energy is a more generic term for mass-energy and momentum. The term derives from the stress-energy tensor, a concept in general relativity: the stress-energy tensor is the total momentum and mass-energy content of a region of space, which in turn determines the metric tensor (the "shape" or curvature of spacetime or the gravitational field) via gravity, hence also the name "metric drive". Metric Drives are not actually capable of generating the titanic stress-energies needed to manipulate spacetime on the scales necessary for high-thrust transport, which in fact requires the equivalent of ultra-dense planetary masses of negative (roughly speaking, antigravity) or positive (conventional gravity) mass. Only Type 2+ Kardashevs are thought to possess the ability to manipulate such energies at industrial scale, allowing them to operate independent of the NSET brane when necessary.
Metric Drives are capable of manipulating spacetime metric, but only on an extremely small basis, too little to provide meaningful propulsion to a spacecraft. At vastly subatomic (Planck) scales, wormholes are generated naturally via quantum tunneling, and the Metric Drive carries the ultra-high precision equipment necessary - the kernel - to extract such a wormhole "seed", stabilize it, and imbue it with a net charge such that it can be confined and manipulated by electromagnets.
Once a seed is prepared, it is known as a controller. One gate (mouth / portal) of the controller wormhole remains inside the ship, while the other gate separates from our brane and from Known Hyperspace, and joins to the NSET brane. The controller can then be oscillated and moved in the NSET brane to "summon" the real stress-energies required for gravity manipulation at scale. This requires complex programs running on significant computing power, which engineers sometimes call the "sigils".

Artificial brane

Many of the unique properties of the NSET brane suggest artificial origin: it appears to be tailored to prevent unskilled users from incurring catastrophic damage within their own branes. On approach to an object of significant mass (even the size of fairly small asteroids), the spacecraft is compelled away from a collision course, or forced to "halt", matching velocity with the object. Potential causality violations are rarely met with catastrophic failure modes; rather, the FTL network undergoes a geographic shift (although this does not mean that such shifts cannot be deleterious to multiversal society in other ways, or to the spacecraft inside). In the event that a catastrophic failure is somehow forced, its effect is dampened from the nova effects that would ensue under "ordinary" conditions.
Artificial branes are a well-known and common phenomenon - a warp bubble can be thought of as an artificial brane, especially when it disconnects from a larger brane to travel between universes in hyperspace. It is known that the Webway War which disrupted the Local Habitable Nexus circa ~2500s AD demonstrated apocalyptic weaponry capable in some cases of modifying physical law altogether, attested by some Kardashev survivors and modosophont witnesses (see Webway War for further reading). Conditions observed in the vicinity of the Metric Ghosts also appear to validate the possibility of such technology, which might make engineering such a colossal artificial brane possible at far greater advancement of the necessary science and infrastructure.