Cyanide

My understanding (and I bet the article above does a better job of describing this than I) is that while other cigar tobacco leaves are fermented in the usual aging process, candela leaves are heat cured. Heat curing is a very fast process, but it gives no chance for the leaf to "mature" into a proper smoking leaf. Thus, its about as close to smoking dried leaves (say off your backyard tree) as you can get.