What is the 'key' to these tantrika works? At the end of this article, First Principles of Buddha Dharma, in the  'Summing Up' section, it says,


  In the sense of Dharma being the 'teachings' of the Buddha, the teachings that explain the Path of Awakening, we find them composed in twelve types of scripture.


and,


  (12) Upadesa, concerning esoteric doctrines, the equivalent to Tantra ('tradition'), in which the Buddha appears in various archetypal forms, for the illumination of beings. These tantrika works are quite numerous, but not readily comprehensible except for those who have the correct key to their meaning.


What is that Key ?
What is the meaning of it?
Is it a secret of that tradition?
From Lotus Sutra:


  He has thus taught this sutra in many places for the sake of the
  sentient beings. Sometimes he has appeared in the form of Brahma. Sometimes
  he has appeared in the form of Śakra. Sometimes he has appeared in the form of Īśvara. Sometimes he has appeared in the form of Maheśvara.
  Sometimes he has appeared in the form of the great commander of the devas.
  Sometimes he has appeared in the form of Vaiśravaṇa. Sometimes he has
  appeared in the form of a noble emperor. Sometimes he has appeared in the
  form of a minor king. Sometimes he has appeared in the form of a wealthy
  man. Sometimes he has appeared in the form of a householder. Sometimes
  he has appeared in the form of a state official. Sometimes he has appeared in
  the form of a brahman. Sometimes he has appeared in the form of a monk, nun,
  layman, or laywoman. Sometimes he has appeared in the form of the wife of
  a wealthy man or a householder. Sometimes he has appeared in the form of
  the wife of a state official. Sometimes he has appeared in the form of the wife
  of a brahman. Sometimes he has appeared in the form of a boy or a girl. Sometimes
  he has appeared in the form of a human, or a nonhuman such as a deva,
  nāga, yakṣa, gandharva, asura, garuḍa, kiṃnara, or mahoraga.
  “In this way he has expounded this sutra and saved those in the states
  of being of hell-dwellers, hungry ghosts, animals, and those in difficult circumstances.
  Sometimes he has transformed himself into a female in royal harems.


...


  If he is able to save them in the form of a śrāvaka, he teaches
  the Dharma by changing himself into the form of a śrāvaka. If he is able to
  save them in the form of a pratyekabuddha, he teaches the Dharma by changing
  himself into the form of a pratyekabuddha. If he is able to save them in
  the form of a bodhisattva, he teaches the Dharma by changing himself into
  the form of a bodhisattva. If he is able to save them in the form of a buddha,
  he teaches the Dharma by changing himself into the form of a buddha. Thus
  he transforms himself in various ways according to the capacities of those
  who are to be saved. If he is able to save them by means of parinirvāṇa, he
  manifests parinirvāṇa to them.