From Halopedia, the Halo wiki
Torsion plates are a maneuver drive system employed almost ubiquitously aboard Forerunner starships. The plates convert harvested energy into a form of inertia-less thrust (presumably torsion fields), and are considered so simple and widely-applicable they are near-universally used across the Forerunner fleet. Other forms of propulsion are used, though they are generally used for extremely specific niche or aesthetic purposes.
Supernal Spiral-class keyships such as Anodyne Spirit are equipped with torsion drivers, as are more terrestrial (though still floating) structures such as skydocks. The accuracy and precision required of atmospheric maneuvering makes torsion plates practically a required feature for any atmospheric flight.
The Wings of Malygus, the maneuver drives of Forerunner origin used to move the city-station High Charity around, were likely torsion plates - owing to the city-station's construction around the remains of a derelict Fortress-class vessel. The Covenant's own repulsor engines are considered a rudimentary step between standard fusion drives and fully-utilising torsion drives.