The plants that were given 72-hours of darkness were not harvested before the period of dark, and that included being pulled from their pots with either a full rootball, partial rootball or mostly bare to bare rootball.
 
The plants were left to finish the last 72-hours of their life, while still continuing to function, in darkness.
 
Plants store energy during the day to be used during periods of darkness. During periods of darkness most plant functions are turned off and the stored energy is used only for the most needed most important functions to continue after sundown or lights are turned off.
 
Since only key functions are performed even though the plants are running on stored energy those functions then get a larger amount, an increased amount of energy to use then during daylight hours. During hours of light plants perform many more tasks and allocate their available energy to more functions meaning less energy for some functions than they would receive when they would be one of the few functions performed during hours of darkness.
 
In very simplistic terms one of Mother Nature’s reasons for THC was to work like sunscreen for the inner trichome head. As light rays strike trichome heads THC works like sunblock but in doing so some THC is broken down, it is lost. During hours of darkness when more energy than during the day is allocated to the function THC is produced to replenish what was lost during the day and then some additional amount is also produced and that is likely greatly genetically controlled.
 
So what you have is a washboard-like uphill line of THC creation, up a little, down a little, back up again and then a little more, down a little, back up again and then a little more again etc.
 
The idea of giving plants 72-hours of darkness in part is found in that means for what is the longest expected or possible amount of time that plants can operate on stored energy and have there still be at least a minimum amount of improvement in relation to the time taken the trichome heads have 72 hours to crank out THC without experiencing the loss normal amount of loss that would have occurred during the same hours if in a normal light cycle.
 
Not all genetics will respond equally as well, and that cannot be expected, but in some strains tested there was as much as a 30% increase in levels of THC found in the plants that where given a 72-hour period of darkness when compared to the same genetic plant grown under the same conditions but that did not get 72-hours of darkness before harvesting.
 
So it is not just a matter of leaving roots on or leaves on. Once a plant is dead, or at least its root system and or stem are damaged to the point being useless and death is imminent all those functions that still occur when a plant spends its last 72-hours living in darkness cease, they stop, they do not go on, they cannot go on so they cannot produce the same effect or a similar effect.
You asked; "If you chop the plant and put it in darkness, doesn't the THC acid keep flowing during the drying process?"
The answer is no. For one THC doesn’t really flow, or at least not in the way many people seem to believe. THC is created in the trichome heads and the only flowing it does is to travel an extremely short distance to a honeycomb shaped subcuticular surface and gather.
 
Again to keep it simple various substances travel up the trichome stalk into the head and interact with light and chemicals and cannibinoids and various substances are created.
 
When you whack a plant what is needed to maintain the delivery of the needed substances to then travel up the trichome stalk to then interact with light and each other is not longer there, that function has ended. Like I used as a very simplistic example expecting those functions to continue after branches are removed from a plant (regardless of leaves being left on or not) would be like expecting the blood in a severed arm to continue to pump into the hand and it being able to continue to function to some degree for some period of time.
 
If you remove a branch the previous transferring of substances halts. The water pipeline has been severed, the electric lines have been cut, the phone lines are down and the cable line is down … however you want to look at it for comparisons sake that is what happens.
 
The most that might be drawn from some very nearby plant matter to other nearby plant matter would be moisture and moisture could contain nutrients but remember you are now in a drying situation, not a growing situation.
You do not want light, the less light the better since light breaks down THC so even if some small amount of moisture could be drawn from a few cells around another it will not draw up a branch and through leaves like a wick once the plant is dead.
All that would happen if some cell or small group of cells did draw a small amount of moisture from surrounding cells would be a more equal overall drying out of the plant matter through evaporation and not that some cell or cells of a dead plant were still functioning and used up the moisture and what might be found in it in some normal profitable gainful way, and that includes any; "flowing" of; "THC acid."