The Gate bearing hangs from the ceiling of the gate room aboard the Destiny. The bearing lights up with each glyph dialed on the Stargate, serving a similar function to that of the chevrons on the Milky Way and Pegasus Stargate designs.
Because the entirety of the Destiny gate spins rather than just the inner ring, the chevrons spin as well and therefore the addresses do not line up with the chevrons to lock. Instead, all nine chevrons light up when the Gate is dialed, and each individual glyph is spun until it reaches the topmost position, at which point the glyph lights up. The Gate then changes direction and repeats the process until the point-of-origin glyph is locked and the wormhole forms. The Gate bearing lines up with the topmost position, and the orb at the end of the bearing lights up briefly as each address glyph on the Gate locks directly below it. Once the wormhole is formed, the gate bearing remains lit while the gate is active. (SGU: "Air, Part 1", "Air, Part 2")
The exact purpose of the Gate bearing is not clear. Destiny-style Stargates on planets do not have them yet function the same regardless, which suggests that the bearing is superfluous where locking addresses is concerned. The Stargate aboard Destiny appears to be the only one fitted with such a device, so it may have a purpose apart from what it does on the surface. One theory is that is where the extra energy needed to dial a nine chevron address goes through to prevent the normal circuits from overloading.(SGU: "Air, Part 3")