in leaving the plant in the dark, you will force the plant to produce more resin because the plant thinks it's going to die. the marijuana plant produces resin as a means to help catch more pollen from the male plant. however if it's not pollinized, the plant keeps producing resin. well when you leave it in the dark it goes into "overload" and vigorously tries to catch pollen--by producing resin.
this statement isnt ENTIRELY true...

Darkness enduces the flowering hormone in cannabis, this is why the lights go to 12/12, when you leave the plant in the dark, it will constantly enduce the hormone, but, without the right amount of stored energy, (from light the previous day) and light to develop the bud after darkness, all you end up with is pre-mature buds. also resin doesnt catch pollen, thats what pistils are for.

if you are thinking of giving your plants and extended dark period, do it a week and two days before harvest, then, a week from harvest, flush and switch back to 12/12 for the last week, that way the extra buds produced in the extended dark period can mature, and any light/energy the plant recieves would usually be used to mature those buds, thus meaning the plant wont create many more immature buds, and the bud will be a little more consistently and evenly grown...

leaving you with more bud/active resin...