Hello to previous commentators,
Just some info--- the only practices I ever heard of anyone of the Ngagpa community doing, and these were Nyingmas of Penor Rinpoche's lineage, were simply high yoga tantra practices. I suppose as a Buddhist, and I consider Dorje Shugden a Buddha, I wouldn't consider Nyingmas (at least Penor Rinpoche's ones) to be shamanistic; they rely completely on a mandala of Buddhas, not spirits, just like all the other traditions. They are considered mystical, by western standards, but they are simply doing high yoga practice with the intention of liberating all beings from suffering. I wouldn't see this as shamanistic, but simply as becoming a Buddha for the benefit of all. Sorry, I don't have sources, because it's high yoga in another tradition and I don't have their empowerments, but I doubt I could direct you to read them, unless you would have their empowerments:).