Gravitational anchoring is the phenomenon whereby objects are held stationary relative to their surrounding environments. While not capable traditional propuslion, gravitational anchoring are ideal for keeping objects afloat. Devices that facilitate gravitional anchoring are also called G/AG/GA or anchor/grav drives.
AG drives manipulate the unusual properties of isolated and tightly-constrained samples of the fundamental particle that is the force-carrier for gravity - the graviton. Specifically, the graviton core of the drive contains a number of particles commensurate to the weight the drive needs to lift, suspended within a spherical shell of graphide that keeps it from experiencing anything of the outside world except through gravitational influences.
Once a drive is manufactured, it cannot be made to lift anything heavier than its rated load unless the alteration is done in specialised facilities - breaching the shell and allowing matter to contact the graviton core causes it to dissolve. While the shell surrounding the core must necessarily be immensely strong to resist the buckling against its internal vacuum, it also means that any breach of the shell will quickly nullify the core's effects.
The graviton core does not exert unusual gravitational properties - such as weakening, strengthening, elimating or reversing gravity in a given location. Rather, when the core is energised, it is made to remain in a constant position and orientation relative to its surroundings based on their mass. Regardless of how much power is being fed to the core, it will remain stationary, but adjusting that power can increase either the range of objects it can lock onto, or the size of potential anchoring points.
For visualisation, imagine that as soon as a drive is activated, an invisible sphere is made around it. As more and more energy is fed to the core, this field expands. The drive will remain stationary relative to the dominant massive bodies within this field - such as buildings, landmasses or even asteroids - however keeping the field small will instead anchor it to lighter nearby bodies, potentially even the object that is supposed to be held aloft, nullifying the drive's usefuleness. The drive requires stable bodies to anchor too as well as massive ones - anchor oneself over the ocean might result in the raised object adjusting with waves or the tides if the field isn't large enough to reach nearby landmass on the surface or at the seafloor.
The drive will never see an attached object move anywhere, and even in the instance of there only being a single dominant body in the vicinity, distance modulation to that prime anchor has not been acheived, if it is even possible. Any object kept aloft by the drive must be placed there or move to that position under its own power. Similarly, attempts at manufacturing asymmetrical cores to target individual anchor points have proven extremely difficult.
Though technically the graviton core maintains relative position and orientation to its anchors, the drive surrounding it can rotate about the core to allow attached objects to change the direction they are facing. For smooth re-orientation, cores are mounted as close to an object's centre of gravity as possible.
Drives are also useful to slow the descent of objects initially moving at great speeds, however sudden activation of the drive at high speeds could result in the drive being held in place while the rest of the attached object is torn away. As such, falling objects are usually slowed through repeated, ultra-short duration activations of the drive. The resulf of this is shuddering vibrations oscilating along the direction of the fall as the subject is brought to a stop. One activity revolutionised by the drives is stealth paratrooper insertions - parachutes cast a large profile, and jetpacks release easily-detected emissions, neither of which a pack-mounted drive does.