Coupe poudre is a poison used to induce a death-like trance in victims destined for zombification by Haitian voudon bokors.
The active ingredient in the poison is tetradotoxin, which is also the poison which gives fugu its potential lethality, although in this case it is culled from a marine worm indigenous to Haitian waters. Tetradotoxin is a sodium-ion channel blocker, which in very small doses leads to a state which Miracle Max would call "mostly dead". The metabolism of the victim is so reduced that he or she certainly appears to be "all dead." Blockage of the peripheral nerves effectively shuts down the nervous system - although the brain and the senses remain alert and conscious. The coupe poudre may also contain hallucinogens, to make the whole buried-alive experience that much more interesting.
The coupe poudre wears off in 10-12 hours. The victim will be dug up at some point before they actually die, and will then be then fed atropine and scopolamine - both dissociative hallucinogens which have an amnesiac effect and a high degree of induced suggestibility. They are derived from indigenous Datura metel and Datura stramonium, both known colloquially as "zombie cucumber." The net result is a broken and highly pliable mind, suitable for further zombie training.