I personally can't see how a few days of darkness can increase the potency, since plants grow so slowly and react as slowly to changes. The only effect I have seen from extended darkness is some stretching. At the rate buds/flowers grow, how much extra weight does a plant add in a few days?I to was taught to leave my plants in darkness for 48 to 72 hours before chopping, the idea behind this is that dark period will frost up your plants, does it work? I dont know, but i do it.
I don't truthfully know the answer, but I sort of classify it with a lot of other "urban canni-legends" or "Bro-science". I would love to see research that would examine it, but I haven't seen anything other than stories from a friend of a friend. I was skeptical about 10K finishing bulbs frosting stuff up, but I'm still on the fence about that one although it is a 2 week process, not 3 days.