Boiling attached Cannabis roots after harvesting whole plants, but before drying, is an interesting technique. Originally it was thought by cultivators that boiling the roots would force resins to the floral clusters. In actuality, there are very few resins within the vascular system of the plant and most of the resins have been secreted in the heads of glandular trichomes. Once resins are secreted they are no longer water-soluble and are not part of the vascular system. As a result, neither boiling nor any other process will move resins and cannabinoids around the plant. However, boiling the roots does lengthen the drying time of the whole plant. Boiling the roots shocks the stomata of the leaves and forces them to close immediately; less water vapor is allowed to escape and the floral clusters dry more slowly. If the leaves are left intact when drying, the water evaporates through the leaves instead of through the flowers.
Whole plants, limbs, and floral clusters are usually hung upside down or laid out on screen trays to dry. Many cultivators believe that hanging floral clusters upside-down to dry makes the resins flow by gravity to the limb tips. As with boiling roots, little if any transport of cannabinoids and resins through the vascular system occurs after the plant is harvested. Inverted drying does cause the leaves to hang next to the floral clusters as they dry, and the resins are protected from rubbing off during handling. Floral clusters also appear more attractive and larger if they are hung to dry.
I may have missed it, but what does adding the boiling water do?