Invigorate is a spell that removes the cooldown of the previously used spell, allowing you to use it again immediately.
The cooldown on Invigorate, without Nanzuum Mana Cube, is 5 hours.
This forbidden ritual instantly restores the previously used spell. As to why it is 'forbidden', we are still figuring that part out.
Using Invigorate after you use Dark Alchemy allows you to double dip on a lot of gold in a short period of time.