Boosted Gravity Propulsion Drive is the boost that is put onto the Covenant vehicles such as the Banshee, the Ghost, the Wraith, the Spectre and the Shadow in Halo 2. The Boosted Gravity Propulsion Drive boosts the vehicle into the air. Some say that is only the boost, but it is not responsible for the flotation system of the vehicle.
Others suggest and entail that there is a drive made and built up of natural Boosted Gravity Propulsion that allows the floating movement of the Ghost and some vehicles, but doesn't allow the movement of the Banshee and the forward movement of the Phantom's aft and body.
This system acquires power from the main engine of the vehicle, and cuts power to the weapon systems, allowing for an increase of speed, like the Ghost and the Banshee.
The Banshee uses this system only when "boosting". When normally flying, it uses anti-gravity pods located on its haunch-sided wings.
The biggest ship that uses the Boosted Gravity Propulsion Drive is the Phantom, that has four protectors on the lower part. The Seraph may use another propulsion system.
The Shade plasma turret has what appears to be a modified Boosted Gravity Drive projected by its base.