The spell is stupidly designed. The burn effect only applies if the full stealth duration has been channeled uninterupted. If you interupt the stealth before the 3 seconds have passed, you will not get the burn effect.
I didn’t notice the cooldown went on a short timer after interupting the channeling. This would make it a good tool to blind your foes – not burn them – repeatedly (blind traits anyone?:) ). This should be its sole usage. It cannot be used for damage, as you do not actually deal more dps by waiting 3 seconds to properly channel it until the burn effect; and for damage mitigation you will prefer the continuous blinding over a few miliseconds extra stealth.