Wormholes are a connection between entrances and exits of one Universe Brane and another. The other Brane is called 'Hyperspace' and cannot be perceived normally by residents of our universe. Phenomenon seen within the hyperspace realm are Plasma and Time Pockets. This is also the realm that Xyrak'l are theorised to originate from.
Physics as we know it pictures Universe Branes as thin sheets hanging in parallel inside Brane Space. This is an oversimplification. If space as we know it is curved, then the timespace Brane is certainly crumpled like a used tissue. Some universes are parallel only where they brush against each other in the great collection of similarly crumpled Branes.
In the case of this universe, our neighbouring Brane is Hyperspace. The areas where spacetimes touch become the entrances and exits to hyperspace.
Begin by picturing two tissues, almost in parallel, but warped and bent by some super-cosmic hand. Where the peak of one touches the valley of another, a 'soft spot' forms that is a passage between one universe and another. In our case, the universes touch in a multitude of spaces. If you flatten out an imaginary Brane, then the relative space between two specific 'soft spots' may be shorter in one universe than another.
When one has to travel at Close To Light speed, such shortcuts are vital.
The known rules of Hyperspace make Gravy Drives unusable within the realm.