What is Demon Soul?

"Demon Soul (level 85): Fuses the warlock's soul with his or her demon. This provides warlocks with a self-burst cooldown to use. The specific effects granted by Demon Soul depend on the demon chosen. Demon Soul lasts for a certain number of charges or until it expires (around 20 seconds), depending on the demon used. 2-minute cooldown."

Does it increase damage, haste? It's so vague!

Edit: I was really thinking about how cool this could be. Imagine becoming your demon and making your body just being invulnerable and you're casting spells as your demon with increased damage and effects such as Felhunter doing interrupts, Voidwalker taunting and absorb, Succubus having a charm that isn't channeled, etc. Like Eyes of the Beast but hardcore.