i was actually just reading about this in jorge cervantes medical growers bible.

"harvest seed crops when seeds are big and ripe. often, seeds actually split open their containing seed bract...once pollenated, the majority of the female's energy is directed toward seed production...seed crops can be left in the ground untill seeds 'rattle in the pod,' but most growers harvest them before then...when seeds are mature, remove them from the pods by rubbing seeded buds between your hands over a container."

page 94 in
Marijuana Horticulture: The Indoor/Outdoor Medical Growers Bible
by Jorge Cervantes

this is the only book ive ever used, hope it helps.