I recently found this article “Atlatl Weights in the Collection of the Royal British Columbia Museum” By Grant Keddie, Curator of Archaeology.
Great pictures of weights in the BC collection … and good answers to the question, “Why are weights used on throwing boards (atlatls)?”
“[T]he stone weight tunes the atlatl flex in relation to the spine of the dart. This results in the dart being propelled foreward with greater control […] The purpose of some decorative atlatl weights is, of course, also symbolic – with meanings that vary among culturals. Some preserved wooden atlatl boards have been found with small ornamental attachments. Some look like mini atlatl weights, but due to their light weight, can only be symbolic or purely decorative.”