I think it fires a missile equipped with an FTL drive from a railgun. Of course if this existed in real life, and was used on Earth, the ecosystem would collapse due to the enormous impact crater, or the planet would just have a massive hole running through it, depending on the impact speed. Either way, we wouldn't be having a very good day.

In-game descriptions and such describe it as a massive nuclear warhead propelled via a railgun apparatus. Given Extended Range Missiles is a requisite research subject, the warhead itself may have its own propellant supply to accelerate itself towards its target once it drops out of phase space.

The warhead's kinetic energy alone is extremely destructive, but adding a nuclear payload to it just makes it absolutely devastating. Effects are known to reduce the overall habitability of a hit planet.