I think what his mentor(s) thought and passed down to him was by putting them into darkness it would shock them into thinking winter is around the corner so start pumping resin and fattening up those flowers so that when they die they can drop seed and those seeds will be at their peak health to get them through winter. I've heard this over the years and seen other growers employing it as they too were told this by their mentor(s). How much validity it has is really dependent on what science says and, as has been noted, science says it has no effect.
We're all susceptible to broscience, me included, but science should be the basis of our knowledge base so, even though it's hard to let go of practices that we believe to be beneficial to our plants we need to be open minded and be able to accept that what we've been told by trusted sources is only as good as those trusted sources ........well.....trusted sources!
Which brings me to this threadjack.....I put my girls in dark for 36 hours before I change up my light schedule to 12/12. Apparently it's supposed to shock the girls into thinking winter is coming and trigger some sort of hormonal change in them that produces more bud sites and gets those bud sites to pop pistil faster. IIRC, I read this in one of those "How to grow weed" books. I think I got this from the Jorge Cervantes book. Valid or Broscience?
OP...short answer? Don't water unless your media is bone dry and I would wait a day for water level in the media to balance out if you do have to water.