"Some Soldiers like to bring an extra Spinfusor as a secondary. This variant has a reduced direct-hit damage bonus compared to Thumpers or other Spinfusors, but retains all other benefits of the disc." In-Game Description.
Like the standard Spinfusor, the Spare Spinfusor fires explosive discs in a straight line over long distances. These discs explode on contact or when their maximum range is reached. Unlike the Spinfusor, this weapon only deals 660 damage on a direct hit (the lowest of any other Spinfusor), and 600 or less splash damage. The advantage however is that the Soldier can also carry his Assault Rifle with the Spare Spinfusor.