The hypothesis goes that darkness before harvest, allows the plant to allocate leftover resources to sink sites. The problem here is that cannabis does not hold any long term stored resources, due to it being a angiosperm (anual plant). It is typically biannual or perennials that hold this ability. There is whats called transitory starch, which are short term energy stores that are used for the dark phase of the diurnal cycle. The problem here is that this store is used to such accuracy, where almost all of this resource is used before the light cycle returns. So 12 hours with 12/12 light cycles. This means if there is any benefit. Then anything longer than 12 hours of dark will do nothing. But it can also be said that even with those first 12 hours, the effect is minimum and if there were more potential to be had, then you would instead be better off continue growing the plant as normally. Rather than harvest.
There is no current theory that supports this, or research data.