Tiina Hyytiainen, of the University of Helsinki, recently conducted anthropological field work on the ngakmas of Repkong, in Amdo.
Repkong has one of the strongest extant ngakpa traditions. (Some may know it as the former home of Lama Tharchin Rinpoche, one of the best-known ngakpa lamas in the West; he now lives in California. This page has history of the Repkong ngakpa tradition.)
Ngakmas (female non-monastic tantrikas) are apparently a new institution there, however. Hyytiainen writes:
Around the year 1988 some [ngakpas] were performing a ritual together, and their wives asked [the head lama] if they could join in the ritual. [He] agreed. This pioneering group had nine women who started and continue to perform religious rituals together, and at present about forty ngakmas between the ages of eighteen to sixty-four have joined the initial group. There is no proper religious lineage to trace back to, however, only a few known examples of ngakmas from the past.
This raises many interesting questions. Hyytiainen asks
… why these ordinary laywomen have turned to committed religious practices and have become ngakmas, and [what are] the effects the religious practices of this kind bring to the overall community.
It would be valuable also to know more about the "few known examples of ngakmas from the past." And were there there were ever regularly ngakmas in Repkong, or elsewhere? Or were they were always one-off exceptions?
An abstract of her work is available; search for "ngakma" to find the relevant piece.