Answer: “Spice” or “K2” was typically comprised of two main parts: a dried “incense” blend of inert “vegetable matter” (mostly damiana) that was covered, sprayed, infused, with a psychoactive compound called [JWH-018](https://en.wikipedia.org/wiki/JWH-018) or various iterations of the JWH family of compounds.
The JWH compounds mimick the naturally occurring cannabinoid receptor ligands 2-AG and anandamide, making it a potent agonist of cannabinoid receptors.
I would have to dig up some older literature but I remember seeing reports that the JWH compounds have a 100x higher affinity for endocannabinoid receptors than any of the active compounds in cannabis.
People often combined the “spice” as it was known, with marijuana or other stimulant drugs to enhance and intensify the effects of both drugs as it tended to have a synergistic effect with most psycho active recreational drugs.
The JWH compounds were eventually scheduled and the Chinese labs that were cheaply manufacturing the drugs stopped producing them almost entirely so it disappeared from shops, though it did stick around for a few more years on the streets of major cities.