I can assure you that it does increase the THC levels. You can think of it this way. The resin is what the plant produces to trap the pollen from the male plant. This is a survival response genetically programed into the plant.
The plant does not produce the resin if the negative phase because the 18/6 light cycle makes it think it is spring or summer time.
The light cycle change to 12/12 makes the plant think it is fall, and time to breed to assure the continuation of the species. The buds form and produce the resin required to trap the pollen and form seeds. If the plant does not get pollenated it starts to produce more and more resin. This is why you will see the resin increase the most toward the end of the flowering phase. It is like the plant is in panic mode trying to catch ant pollen it can. Now if you turn the lights off totally for 3 days before harvesting, the plant puts all possible energy into this last ditch effort to be pollenated.
Now I also no people that have tried increasing the periods of darkness from 12 to 14 to 16 then 18 hours to push resin production. The plants only goal is to reproduce, and senses the increased darkness as winter and the end of its life.
The down side of this process is that you risk the plant becoming a hermaphrodite, and pollenating itself. That is why I only black them out for 3 days. It is not enough time for the plant to herm out.
I also cant give you a number that this increases the THC level to, but it does increase
Sorry this response got so long
Scooter