The dry spell right before harvest is to increase the THC content to .. pretty much as high as you can get it. when the plant has a dry spell it soaks up alot more of the moisture in the buds.. mildly increasing the THC content. Increasing the dark period signals to the plant the days are now getting much shorter, and it's time to bud like mad.. because soon, the plant knows it will die. On my last grow i put the light cycle to 10/14 as opposed to 12/12 for the last week. then 8/16 for the last few days. every morning when she awoke, there were new hairs all over(and that's late in the flowering cycle). Proving that the light cycle DOES make the majority of the difference when harvesting MJ.

This is all pretty much straight from Mel Frank & Ed Rosenthal's "Marijuana Growing Guide".