It does seem to be a part of some/many folks finishing philosophy, one I don't employ. I might let my plants thirst some just before harvest, but the darkness thing doesn't make any sense to me.
Seems it'd tend to cause the buds to s-t-r-e-t-c-h and thus, loosen, but I wouldn't know for certain,
Yes you want to cut the water out at the end. As for the darkness we've never tried it but it seems that a lot of people do it and sware by it. So it's up to you but before you deceide make sure you did enough research to make a good desicion.
The dark thing is bull! That is just a May J myth! Plants are all set up genetically,thus if a plant is in flower then it will continue to flower until finished. This includes potency too. Now for the water part i always water up until the day of harvest, although a plant is made up mostly of water you can stop watering maybe a day or two before harvest to help the buds dry a little faster than to transpire excess water in the branches and stems.