Although this is just a theroy, I am under the impression that the resin a cannabis plant produces is to protect the seed from drying out by stoping some of the moisture from evaporating. Many plants have resin that acts in a simmilar way, but lack the chemicals that we enjoy in cannabis. If that is the case, then letting the plant become a bit dry before harvesting could increase resin production.

Another theroy is the fact that light degrades THC. Maybe the resin protects the seeds from UV light; degrading durung the day and producing during the night to prepare for the next day. Therefor, if you were to harvest right before dawn the resin production would be at it's max.

My 2 cents.