With the Float Burst, Plague Knight floats down slowly after the explosion. This makes it ideal for new players still getting the hang of the Bomb Burst. Its downside is that Plague Knight cannot charge a new Bomb Burst while floating, making it less effective for advanced players. However, pressing down while floating will allow Plague Knight to return to his normal fall speed and enable him to charge another bomb burst before touching the ground.
When combined with Vats the Float Burst allows Plague Knight to remain in the air for long periods of time.