Actually it is not the spell but the small negative casting light in your hand(s) that causes this phenomenon.
The spell itself is merel a byproduct. Just a one minute duration nerfed version of candlelight spell.
It is even possible to use other lightsources like spells or enchantments and negate their light level with this.
The whole thing was inspired by Lertkrush/Kittytail's Moonshadow Spell from The Art of Shadow.
So all credits go to Lertkrush/Kittytail, I just tweaked some values and did some testing.

So you can think of this as an alternative to nighteye without their weird imagespaces.
You can use it by carrying the spell in your free hand(s), somewhat like an illusion concentration spell without magicka cost.
Double-(not)casting decreases the light level even more.
Oh - works most reliable in interiors, as exteriors update their light level recognition muuuch slower. Took up to 20 seconds in the testruns on my last over-modded setup, my new vanilla-ish just needed one second.

Recommendations:
Go get The Art of Shadow.
That one has two more spells, scrolls, a staff and you have to find or buy everything - it just has no Sneaklight.
Highly recommended for daylight-sneakers.
ENB is highly recommended, as without your character will be really dark at the front, thanks to the handheld negative light(s).
I further recommend some better casting animations like this, this or this. My personal favorite is the first one.
The vanilla animations are just ape-like in third person, not worthy of a real mage.

Have fun