This question was asked the first week the game opened. Tokesalot said he asked the Devs what that means, and they don't know. Nothing has been clarified officially since that time. In all honesty, it's kind of scary when the devs of a game don't even know what the things do that they put in the game.
It expands damage thus meaning it may give an increase on damage on a hit or even decrease. The following that I am about to paste may help better explain....
Astral Floating Damage Expands Analysis
Based on my Wartune damage calculation guide, you will know that the damage calculation in Wartune is centered around the medium damage by 10%, which makes the actual damage 90~110%. The Astrals that affect the floating damage expands those limits. For example, a floating damage expand of 20% will make it 70%~130%. Because of this, this astral is useless at lower level. This means that over many battle turns, this astral will help you exactly…nothing.
However, this astral has a powerful affect if you are a critical damage player plus critical enhancement. Let’s simplifyy the battle scenario: Let’s say if by average you can kill an opponent in 4 attacks – regardless of the skills used. On average without the damage expand it will take you 3~5 attacks. However, with the damage expand, you can widen the number of rounds to 2~6 attacks. How does this help? If you are lucky, you can quickly wipe out the enemy forces and have a higher chance of winning. This means faster kills.
Another way to look at this astral. If you can kill an enemy in 2 rounds instead of 4 rounds, you double your killing speed. However, if you get unlucky and kill in 6 rounds, you only lose 50% of the battling time. Overall, when this astral is mastered at high level with the orange astral. It can be a formidable tool that can 1 shot kill some of your opponents.
This is simply one players explanation which is both unproven and unofficial.
As I said on that post at the time, if this is indeed what it does, it is the most worthless astral to date as the law of averages dictates that the average damage will be identical to your current average damage which makes it's net effect, 0.