Cannabis plants are photo period plants, not counting auto-flowering strains of course, and when the amount of daily light grows shorter, 12 hours or less, hormones are released to induce flowering. The idea of a period of darkness is to speed up the release of the hormones that begin flowering since it is not something that fully occurs on the first or second day where plants receive 12 hours or less of light, it takes a bit of time. The idea is to speed it up, to give it a jump-start of sorts.
I have never attempted it so I cannot speak from experience but I have read where some people claimed their plants began to produce flowers faster and others have claimed there saw no speeding up of flower production at all. It might possibly be something that falls into the category of strain dependent reactions and it might be a myth, an urban legend of sorts or old hippie folklore but if it works at all it appears that it does not work for all strains, or at the least not equally well for all strains, and possibly not at all for others.