The Sunfury is a Flail weapon that can only be found in Shadow Chests in the Underworld. It is stronger than the Blue Moon but weaker than the Dao of Pow. Despite being classified as having a slow speed, its actual speed depends on how far away your target is. When the ball goes too far from the player or hits a block, it retracts instantly. Using the Sunfury at close range is a decent strategy since it will retract on impact and can be used again immediately. This weapon, along with other flail weapons, is not affected by an increase in melee speed.
You can hold down the left mouse button to keep the Sunfury from retracting, and it will repeatedly retract towards you. To let it return, let go of the left mouse button.
There is another way to use the "retract method." If you turn around when it's coming back to you, you will swing it the other way; you can do this an infinite number of times, as long as you time it correctly.